looks good... don't know how long you want to keep the system... but you might look into upgrading to a quad core if its for any length of time....
also you probably don't need a 750 watt power supply... a good 550 w one would do fine
the video card... i'd honestly get the 4850... its very similar performance and has the edge in newer games... and try to get an intel chipset like a p45 or something... you don't need an nforce chipset... all i've heard is bad things about them... even though mine has been fine (680i)
also maybe go to 4 GB? not necessary at all but w/e just my opinion =D
and lastly... you need an operating system unless you like just building computers w/o having them run =P
Where are you buying from Archix? In Canada, NCIX.com is like our Newegg - Except they price match very well! Use shopbot.ca to look around for the parts you pick, then on NCIX's site pricematch them to the sites you find via shopbot
I'm not sure about your memory situation... Do you want a total of 3gb? I'd try and buy the same brand if I were you and not mix and match. Also, I'd go for a 2x2gb kit of DDR2-800.
I noticed that Samsung is the 5ms model. The 22" 2253 model has a faster response time of 2ms and can be found in Canada for around $270 (directcanada.com)
I'm not really a fan of nVidia motherboards... Do you plan on adding a second GTX down the road (SLI)? If not, I'd go with a P45 motherboard - Unless you'd be interested in going with ATI cards, like two 4850's or a 4870 and then another 4870 down the road - then I'd get an X38 motherboard!
I believe that Heatsink got a good review here on Toms. I like the Kingwin or Xigmatek personally, don't have experience with Coolermaster one.
You can save a few $ by getting the Coolermaster 690 case. I love that thing
If you don't plan on adding more video cards, you can go down to a lower wattage PSU like Thogrom said. Corsair, PC Power and Cooling, and Antecs are my poisons of choice.
And Operating sytem? If you want to get 4gb of memory, I'd probably get Vista 64. It's that or XP (32 bit). If you'd like to upgrade to 8gb of ram down the road, Vista 64 for sure.