HD 3550 is DX10.1 card! Commendable performer, i say! On the other hand, 8600GT is an amazing overclocker and considering its VFM, it is best option available to budget gamerz! And yeah, HD3850 will easily crush both 8600GT and 8600GTS! And btw, 8600GTS is more of a dudd s%^t piece than a GPU!
I get 18-23FPS at average with all settings very high and Natural Mod! GPU Clocks are 725/930 24x7 !! One more thing, go for a motherboard brands like Asus and MSI! They provide extensive support via forums, regular updates and loads of features! And yes, avoid P5N-E SLI! I have this mobo and it suffers badly with heating and RAM issues!!
MSI P35 Diamond/ Combo/ Platinum should be the optimal choice!