An HD4350 is the best choice for a cheap, common card that has as flexible a dot clock as possible, and it is available fanless for a quiet cab too. While it is powerful enough for MAME, you will have to ask in the GroovyMAME section if it will be enough for your non-MAME applications. My guess is that you'll want something faster, but that's just a guess. If you just want to test one out they can be found on eBay for about $20.
The fastest card available for CRT_Emudriver is an HD4890. That's a lot more expensive though, so I'd definitely do the research before picking one of those up.
How are you going to run CRT_Emudriver and 800x600 in the same machine? Do you have a multi-sync monitor with a range from 15kHz to over 31kHz?