On a budget and got an AGP slot? Look for an ATI Radeon 9700 Pro or
similar. I've got an All-In-Wonder version which I picked up from eBay
a year or so ago for £50. You'll probably find them cheaper now,
especially without the TV card bit. It's fully supported in Vista (and
ATI give enhanced drivers). I can't remember the complete scores, but
the 'desktop graphics' score was 5.9 in my PC!! Still only get a 3.x
score as the CPU and RAM is the limiting factor. This was with the ATI
drivers - the MS included drivers were more like 2.x or 3.x score on the
'desktop graphics'.
A quick search on eBay shows that 9700 Pro's go for about £20-£30.
3D wise, it plays Half Life 2 brilliantly on my 2.2GHz Athlon XP (not
tried HL2 in Vista yet).
So, if you're looking for a basic full Aero support graphics card with
reasonable performance and at a low price, you probably can't go far
wrong with the Radeon 9700 Pro.
Hope that helps,
David