I recently Up graded my PC from a Core 2 Duo, 2 Gb of ram and a gigabyte motherboard to a Asus X58 Sabertooth, Core i7 950, and G.Skill DDR3 1600 6 GB triple channel memory.
so my specs go
Windows 7 64 bit Home Edition
G.Skill DDR3-1600 6GB Triple Channel memory
Asus X58 Motherboard
Core i7 950
Asus Xonar STX sound card.
Instead of getting an improvement I noticed games like crysis slow down to barely playable framerates in action sections which I never got before, also i noticed a performance decrease in New Vegas such as stuttering when i turn around when out in the wasteland again i never had that problem before.
New Vegas has issues with it as it is. There are mods and such on the Fallout New Vegas Nexus that you can use to help you game along. There was one called "No Stutter" that seemed to fix a few issues I was having. I have been running it in 1080p with all settings to max so this may be more my issue than just the game itself.