Having just gone through 2 mobos and 2 CPU purchases and upgrades due to NOT deciding at the start what emuulators I want to run (ooo... I can run PS2, you say... ) I can shed some light.
Tried running PS2 Emu on Dual Core with 1G GPU and 4Gb RAM (the ram doesn't really matter - as PCSX2 is mainly CPU heavy) - can just about get it there on E8500 Overclocked to 3.7, but it is touch and go.
Demul is useful for Dreamcast, Naomi 1+2and another Sega system (which I can't recall). I tried using Demul for lightgun games on dreamcast, but doesn't run well on XP - new version needs Vista and upwards for the new direct X.
Moral - if you're looking to build a system that can play PS2 games and also accommodate future emulators (Xbox or future arcade boards) then go for newer mobo, with i5 or i7 CPU and fast RAM. If you'recontent with PS1 level games and below, then your setup should be fine.
Me, I bought an i7, new mobo and 6GbDDR ram two days ago. I want the lot! And I want the next lot!

Specs here:
https://sites.google.com/site/alienarcade74/project-updates/oooops-anothervisittomrvisaforupgradeI know I will just want to keep expanding this thing, so best to invest at the start by my reaconning.
I now have 4 spare mobo, cpu and ram combos just waiting for new cabinets!! Maybe I should sell them and try and claw some money back from this expensive adventure

hope this helps.