if you're only willing to spend $100 on a machine then I'm not sure it's actually worth upgrading at all, you're unlikely to be able to buy something capable of running much newer versions, so unless there's something specific you're unhappy with now you're probably better saving for a while until you can get a more significant upgrade otherwise (in terms of the emulation you're getting) you'll be more or less replacing like for like...
unless stuff in the 3ghz Core 2 range goes for $100 these days? maybe used.. but then you might end up having to spend a bunch more on maintenance, and upgrades / fixing of older hardware tend to be more expensive because the parts are out of production. We're already at the point where RAM for my Core 2 is more expensive than larger amounts of RAM for newer systems if purchased new.
a lot of interesting classic era stuff has been added since 0.98, be it rare Data East cassette titles like Manhatten, or long-lost prototypes like Planet Probe (which probably only got canned despite being finished due to the developer going bankrupt or similar) but general requirements of MAME are quite a bit higher these days and I'm not sure $100 is going to buy you something decent enough (you absolutely want to be avoiding onboard video solutions etc. as many of the intel drivers are so bad they struggle even with the MAME internal interface)