The trick to keeping a build in budget is patience.  You know exactly what you have to have, i.e., operating system.  Get the OS you want OEM.  Then think about what your computer is going to be for primarily.  If you want something to do basic functions with the ability to upgrade down the road, you can save more but if gaming/high performance is what you are after, expect to pay a price.

My specs are as follows:
BFG 680i SLI
Q6600 SLACR G0 Stepping
Zalman cooler (blue led)
850w PSU
2x 8600GT 512mb gddr2 Video cards on SLI
4gb PC8500 (1066mhz) Ram
1x 250gb HD
2x 160gb HD
DVD/CD +/-RW Drive
CD/RW slave drive
A: Drive
Windows XP Pro sp3

My tower was built for 700 bucks over the course of a few months.  It takes a lot of patience and a little luck to get the most out of your money.  If you have an old computer, that's a good place to start.  Scalp whatever you can from it IF you are no longer going to use that computer.  Inspect the MB, Processor and the case itself.  The HD may be useable.  Take your old hard drive in and have it "ghosted" to a new drive.  You will be asked to re-activate your OS (by typing in the key located on the side or rear of your old case) when you turn your new computer on.  If you don't need something with a window or fancy, use your old case.  Remember, Hobby Lobby has items to help you customize your old case.  A small hack-saw, a little plexiglass and a few stickers go a long way.  In Houston, we have a place called Directron (google the website) which is kinda like a pick-a-part for computers.  I got my 60 dollar cpu cooler there for 20 bucks.  Their showroom is filled with shelves and boxes of stuff for less.  I also got my Ahead Blue Steel case for 20.  Newegg, Tigerdirect (be careful of their refurbished items) and craigslist can be your friend.  Always check out your local craigslist, you never know what you'll find.  I got my Q66 (still sealed in the box) for 150.  Watch the local deals and websites.  Look for rebates, just because a website doesn't say there's one, doesn't mean there is one.  Circuit City is going out of business now so it would be a good chance to get a few items if there are any left.  Oh, and FILL OUT THE REBATES.  Don't just leave them lying around until it's too late.  Holidays are another great time for cheep supplies.  Expecially around Halloween.  There's 4 great events after that.  T-Day, Black Friday, Christmas and New Years.

OS: You can not scate on the cost unless you ghost your old drive to a new one
RAM: There are ALWAYS good deals on ram.  2g kits for ddr2 can be as low as 15 bucks  (newegg.com black friday '08 w/mfg rebate) (30 bucks microcenter sale)
MB: Newegg is the only trusted place i have for mother boards.
Processor: craigslist or net.  retail stores are usually too high but don't rule them out
Case: Your old case or a cheep new one on line would be your best bet, some are pretty expensive
PSU: Try everywhere.
V-cards: do NOT get these off CL unless the description says new in box or garanteed to work.  Newegg these.
HD: you can try pretty much anywhere, ebay, cl, newegg, etc.
Cooler: you might spend a pretty penny here.  there are not usually deals for cpu coolers unless you are wanting watercooling.  then again, you're paying extra for that and the discount is not that much.

Your CRT or old LCD will be good for now.  There are NO good deals until black friday.  My 22" LCD was 149 even, no tax, free shipping.  Newegg is awesome.

Watch for my next post where I will explain configuring your new build.