my first gaming build was on a strict budget just like yours. ~$400 and i did pretty good. I set up a computer good enough to run UT2003 maxed. HOWEVER, the trick is to use as much of your current gear as possible. use the same case (if it fits), use the same monitor, mouse, and keyboard. Use the same hard disk and CD-ROM you have now, and buy one of the retail AMD Athlon 64 processors (meke sure that the processor fits in the mobo. if it doesn't, your screwed). After you have the guts of the computer all set up and working, THEN buy the fancy case and peripherals.
if you do that, your computer may not look good, but it will be much faster than if you didn't skip on the pretty gear.
Best of luck!
BTW: the reason i said get a retail A64 CPU is because the chip/heatsink combo is often cheaper than seprate units. And also use your current OS if its windows 2000 or XP. that alone will save you at least $100