Pretty decent build. I wouldnt roll the dice on the Rosewill PSU though. Not saying go out and get a million watts, but I'd posit getting a bit better quality PSU, take care of that fine other hardware. The Rosewiill is at the bottom quality wise: http://www.eggxpert.com/forums/thread/101922.aspx . For around the same cashie, OCZ StealthXStream OCZ600SXS 600W ATX12V / EPS12V Power Supply. 5 more, much better. Personally I would dedicate a bit more $$ to a PSU, but thats me.
I would also look at a P45 board, there's some decent prices for the same money as that Gigabyte. I'd even be good with a newer P43.
You're right too, that HD is a rooll of the dice. If you dont know, Hitachis run hotter than any other drive. Heat means shorten life. There's a Western Digital Caviar SE16 WD3200AAKS 320GB 7200 RPM SATA 3.0Gb/s Hard Driv for the same $$$, but I'd posit a SAMSUNG Spinpoint F1 HD502lJ 500GB 7200 RPM SATA 3.0Gb/s Hard Drive - OEM for $10 more. F1s are sweet sweet sweet.
The HD4850 is certainly a sweet choice, especially since I'm gonna order one myself this weekend. If you're all for the Sapphire card though, DO NOT expect the rebate back. myrebates411 is THE WORSE company handling rebates around. How Sapphire went back to this guy is beyond me. They have ripped so many people off it isnt funny. Read this guy's (its a one man operation from what I understand) BBB issuance: http://www.labbb.org/BBBWeb/Forms/Business/CompanyReportPage_Expository.aspx?CompanyID=100035461 He's got a grae of "F: for a reason. Read about them at the RebateReportCard: http://www.rebatereportcard.com/rebateratings/reportsearch.php?companyname=MyRebates411. Serious there friend, wanna avoid a headache, get another make. I'm going with Gigabyte myself.
Other than that man, build on.
Penewah!
#1:MSI K9A2 790FX/AMD Phenom 9950BE125w OC 3.12/Xiggie120/Antec1200/HD4870 1GB ASUS/mushkinBlack 4GBDDR2800 /Sammy Spinpoint 640GB&320GB-SATA/PCPC 750W CF Silencer/ LITE-ON & ASUS DVD±R/Vista 64-bit/Dual 22" HannaG Mons/ WD My Book 500GB/Logitech X-530 70 watts RMS 5.1 Black Speaker System /#2:Biostar 780G/AMD 6000+x2/OCZ PlatRev2 2x1GB DDR2800 /Logysis-Acrylic/Vista/Ubuntu 8.1 / Corsair 550W PSU/#3:E8500/Gigabyte P45/HD4850/Xiggie 120/OCZ StlhStrm 700W, CM HAF case/OCZ Plat 2x! 1066