Currently I have Core2DUO, E4400, ASRock mainboard, 2GB ram, ATI RADEON x800 XL, 2 HDD - 80GB/320GB, 550W power suply, x360 controller. This machine is very good for playing emulations (I don`t play PC games). If you know what can I upgrade for better performance post it 
Here is the emulators stats:
MAME (all) - Very good.( 3D games SFEX 2, Tekken 3 slow)
ChanCast - Average (some games slow, some fast)
NullDC - Very good (most games 1:1)
SS - Very good (emulator runs 1:1 or does not run at all, Guardian Heroes stops after the openine movie)
FBA - (CPS3) - Frame skipping
FBA shuffle - Good (CPS3 bad sound)
VBA - Great
ePSXe (with Pete`s shader effects) - Great, no slowdowns
PS2 - very slow, 5-10 FPS, KOF - 30 FPS... Final Fantasy 12: 10-14 FPS... unplayable
Project64 - very good. Glitches in some games. No slowdowns.
PSX - good, 1:1 ps emulation
FCEUltra - great 1:1 emulation, if not better (all image enchancements)
ZSNES - great with all AA, eagle engines and all
WonderSwan - Very good.
PCengine - very good.
NDS emulators - I wasn`t able to find any working NDS emulator.( I am lame I guess )
Here is the emulators stats:
MAME (all) - Very good.( 3D games SFEX 2, Tekken 3 slow)
ChanCast - Average (some games slow, some fast)
NullDC - Very good (most games 1:1)
SS - Very good (emulator runs 1:1 or does not run at all, Guardian Heroes stops after the openine movie)
FBA - (CPS3) - Frame skipping
FBA shuffle - Good (CPS3 bad sound)
VBA - Great
ePSXe (with Pete`s shader effects) - Great, no slowdowns
PS2 - very slow, 5-10 FPS, KOF - 30 FPS... Final Fantasy 12: 10-14 FPS... unplayable
Project64 - very good. Glitches in some games. No slowdowns.
PSX - good, 1:1 ps emulation
FCEUltra - great 1:1 emulation, if not better (all image enchancements)
ZSNES - great with all AA, eagle engines and all
WonderSwan - Very good.
PCengine - very good.
NDS emulators - I wasn`t able to find any working NDS emulator.( I am lame I guess )