*shrug* Mame relies mostly on processor speed (for the most part -- now with the fancy d3d code they are working in not sure how that is changing)
The extra ram (512 is puhlenty of breathing room in this case) will benefit the inital overhead of XP/operating system.
You're probably fine with either sized card... MAME doesn't really soak up video memory in the same way as a 3d PC game... (although zinc on hte other hand...) so 32 is fine...
Are you outputing to an arcade monitor, TV, or PC monitor? That will have a greater effect on your vid card choices.
1. realize with that processor you'll be able to play all classic games thruough neo geo games and I imagine some modern fighters with some frameskip. The midway games like Mortal Kombat might give you trouble, as will nba jams, etc... no chance in hell of playing killer instinct, area 51 etc etc...
2. The best think you can do is work on tweaking all the unnecessary crap out of windows XP. i.e. disable un needed services (what the hell do you need fax service running on a arcade machine in the background for?) I recommend some time checking out
the black vipers page on this*shrug*
Rampy
The only other thing you could try and it would be an interesting experiment with your machine being right on the cusp of break even point... is running it in pure dos *Shrug*