you can probably get a socket 754 Sempron in that price range, although I'd recommend spending the extra $20 or so and getting a 3000+ socket 939 chip as it'd give you more room for upgrades in the future.
You can find a 3000+ cheap used nowadays, and a socket 939 board can be had for similar prices to other motherboards.
For the slightly higher price, I'd say the Athlon64 is worth it, although if you are only doing MAME, you don't really need that much power, and you could just grab a cheap computer based on a AXP 2500+ or similar.