Never get a new Microsoft OS until after service pack 2 unless it is bundled in a new computer. stick with XP, and if you dont have a home network you can stick with the home version, its the same but without some networking utilities, and there is freeware versions of those that are as good if not much better.
Im glad to see you are going with Nvidia, but may i suggest not going with AMD. good processors, but AMD is not doing anything good since the 64 and looks to be that way for the next 2 years. also that socket is being phased out.
http://www.overclockers.com/home.asp
Read whats coming up and think about getting a mobo and processor that has future upgradeability. If AMD was coming up with something that will compete with Intels quad core then I would highly suggest going with AMD and not intel, but for the next 2 years its Intels ballgame, and in 2 years it could be a good time to swap mobo and processor for a newer one.
Asus P5LD2 SE/C Socket 775 Core2Duo ready 945P PCI-E Onboard Audio ATX
http://www.ebuyer.com/product/123821
Intel Core 2 Duo E6550 2.33GHz 1333FSB Socket 775 4MB Cache Retail Boxed Processor
http://www.ebuyer.com/product/130485
Dont let the speed fool you its faster than it sounds, equal to my P4 3.2 Ghz, and excelent overclockability. Upgrade to penryn next year when the 4Ghz is out?......
Its your choice realy, and im only making suggestions here.