all you do is disable the onboard video in the motherboards bios. really easy to do, takes all of maybe a minute (if you have to dig through settings and menu's in the bios)
Otherwise the build list looks good.
much much better list of parts than most 1st time builders present on here.
looks like you did much of your research ahead of time before picking out parts. everything matches up, good quality PSU, etc.
looks good to me.
and Welcome to TechIMO!!!
Only change I'd make is upgrading to a 64-bit OS, like Vista 64. you won't be able to use all of your RAM with XP 32-bit, can only address 4GB max of everything, and you have 4GB RAM, 1GB Video memory, plus whatever cache memory is on the CPU, DVD Burner, HDD's, etc, deduct all the latter memory from the 4GB max, and thats roughly what you'll have for available System RAM.
grab 64bit and you should be able to use all of it.
but even still, under 32-bit, 3GB or just under it will still be plenty for a decent gaming rig.
right now, depending on which version of Vista you get, you can get a Voucher for free upgrade to Windows 7 64-bit when it comes out, right now Windows 7 Release Candidate, though has some hiccups, is almost as stable as current versions of Vista. so when it comes out in about a month and half to two months, it should be relatively rock solid as compared to when Vista debuted.