If you just have to have your own PA, here's what I'd recommend as a good system for the average band.  (Note: Some of the gear listed has been discontinued and replaced with other models that I have yet to hear.  But this will still give you an idea of what it takes to get the job done).

Top Box: 1 per side JBL MPro MP412.  Rated 350 watts, 8 ohms, -3dB@67Hz.  This is a very good sounding box, and properly high passed will put out substantial level with good quality.  (The upgrade from this would be the Yorkville U15, Rated 800 watts program @ 4 Ohms.  The U15s sound very good*, array very well (if you need two per side for wide coverage), and play pretty loud.  (Although I have never heard them, I expect that the Peavey QW3 is a very good box as well - Rated 2000 watts program.)

* The U15 does have a bothersome upper bass bump at around 190Hz (if memory serves).  This is compensated for in the Yorkville processor, but just lightly filling the box with fiberfill helps a lot.  Many speakers have similar problems caused by a lack of damping material (especially the Yorkville E-12, which is a very nice monitor).

Amp:  For MP412s use a QSC PLX3402: 700 watts @ 8 Ohms, 1100 watts @ 4 ohms.  This will handle 2 pr. of MP412 fairly well.  (A caveat: using two JBLs per side will play louder and give more coverage, but said coverage could be somewhat uneven - this remains to be seen).  One could save a bit here by running the pair of MP412 in mono with a bridged QSC PLX1602: 1600 watts @ 4 ohms.)  Or use one side of the PLX3402 for the pair of MP412 and the other for a monitor mix.  For the U15s, a PLX3402 will be just fine, as would a PLX3002.  (For the QW3, a bridged PLX1602 per box should work.)  (These amps have recently been replaced by the PLX2 series.)

Sub: Peavey QW218.  2x18", Rated 1600 watts, 4 ohms.  Don't let the brand name run you off.  This box was very highly rated at the NY subwoofer shootout.  Start with one (one 2x18 is cheaper than a pair of 1x18, because it costs almost as much to make the little box as it costs to make the big one.  The second driver is the major cost factor between the two).  You can get a QW218 for $1150 including shipping.  This is likely M.A.P., so you may be able to get an even better deal if you call around.)  Yes, using one box is ok.  To see why, read about The Power Alley and Boundary Cancellation.  (A pair of JBL MP418 1x18" would be good, as would the QW118 or a single SR4719x 2x18", but the QW218 will cost less and will likely perform better.  Plus, the 2x18 is easier to get into the van by yourself - just tip, lift, slide.)  (If you have substantial woodworking skills, build a pair of LAB subs.)  (If you have the bucks, the Danley TH-115 is likely the ticket.) 

Amp for sub: Crown CE4000: 1 per box, bridged, 3200 watts @ 4 Ohms.  (For LAB subs, I think a Bridged Crown K2 would be fine.)  For the MP418s, use a CE4000 in stereo.)  (This amp has recently been replaced by the XTi400 - $750 new - quite a value.)

Crossover/EQ: dbx DriveRack 260 .  The DriveRack PA is a lesser cost option, but it has a number of shortcomings.  The Behringer DCX2496 in consort with a Behringer digital EQ  is another affordable option that is more capable than the PA.  The best value is likely one of the Peavey VSX series processors.  You'll need a Laptop or rack-mounted computer for maximum usability of most of these units, but it's not mandatory.  Start with crossover of 100Hz between the tops and subs.  Put a high-pass of about 35Hz @ 24dB/octave on the subs.

Mixer: Allen & Heath MixWiz3 16:2 @ about $1000 (older models are available used for $700 or less).  Much better EQ than a Mackie VLZ or SR (the Mackie has groups, but you can live without them).  If you have to go new, the MixWiz3 16:2 and the Mackie Onyx are both about the same price - take your pick (the Mackie has groups, the A&H will be easier and cheaper to get fixed once the warranty runs out).   The Crest XR20 may be even better, but it has fewer mono channels and is a fair bit more expensive.  If 16 channels won't do, the Allen & Heath GL2400 (or a GL2200 - available used in the high teens) is the ticket (it's easier to service than the Soundcraft LX7, and has less confusing routing and monitoring functions).  An upgrade from these would be the Crest HP8.  If you have some big bucks, APB is the way to go.  Digital mixers have a number of advantages over the above models (better EQ, save/recall of mixes, built in compression and effects, etc.) but having no experience with them I have no recommendations for or against (except for the Yamaha M7CL - if you have $20,000 that is).

Vocal Mics: The Sennheiser e835 at three for $200 (or less) is a good way to go.  I prefer the 835 to the SM58.  My favorite is the Audix OM7s, which can sometimes be found for $125 to $150 on eBay or from Rat Sound.  They ARE worth the extra money.

Instrument Mics: The Audix D4 will do a nice job on just about anything.  Often available on eBay for $100 to $125.00.  For kick, splurge and get an Audix D6 or a Shure Beta52 on eBay (if the kick is not well damped, I'd forgo the Beta52 unless you have a really good gate for it).  (If money is tight, check out the EWI mics.)  The Audix MicroD and ADX20D are also very good on about anything other than kick or floor tom (and they are very small, which can be a great advantage on rack toms and snare)  (In addition to racks and snare, I use them on guitar cabinets.)

Cables: EWI, or  make your own.  Get your Neutrik connectors from Full Compass.  Horizon Lo-Z1 mic/line cable is good stuff, but MusiLux is really nice (and comes in all 10 standard colors, unlike the Horizon).  Get your 12-4 speaker cable from Camel Traders.  For power, see my AC Cables Page.

Snake: EWI products are a very good value.  Or you can get a deal on a used Whirlwind, Horizon, or Rapco on eBay.