Thats a really differcult question as every game is different in terms of how CPU dependant it is, The extra cache on the Core i7 will certainly help and so will the extra cores in games that use more than two.
DDR3 Triple channel RAM will give a small performance boost over DDR2 Dual channel.
I'd guess that in games that only use 1-2 Cores and arent memory intensive the E8400@4Ghz would have a very slight advantage (maybe 0-3%)
And in games that are extremely CPU dependant like BFBC2 or Starcraft 2 then Core i7 950@Stock would have a significant advantage.
All X58 Motherboards are very similiar, if your into heavy overclocking then the more expensive boards (ASUS Ranpage III Extreme, EVGA Classified ETC...) will help gain a few hundred more Mhz from the CPU
Other differences in X58 Boards are things like SATA ports and USB 3.0 ETC....