Hello,
I built a Mame in a box - that plugs direct into a TV, and started off like you with a Riva TNT. TV out on it didn't work right, and since it was Sunday, popped over to PC World and picked up an AGP GeForce 5200.
Usually I'd not touch the lower GeForces with a bargepole for any serious gaming (certainly nothing that requires Pixel and Vertex shaders), but I'm massively impressed with it.
Works fine for Mame (baring in mind the rest of the PC is an antique, think it's a P3 600) and no-trouble with drivers. As a bonus the TV out is really nice too - it's only got an S-Video output, but we rigged up a splitter to do both composite and S-Video.
Think it did me for £40 (like I say, it was Sunday and since the rest of the machine worked...) but you'll likely pick one up way cheaper online. Ebuyer in the UK lists this:
http://www.ebuyer.com/product/136164 - which is more like £17.00 - so pretty much half the price.
So err yes. If you're not looking to go mad with the cost and not after serious 3d performance (which I don't think you'll need for most Mame stuff), I'd recommend a GeForce 5200.
Hope this helps!
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S0L
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