While it really depends on what you're trying to do with this system, with only $100 and only a 220 watt power supply (
source), I'd do a RAM upgrade (per the specs 4GB is your max), and then research your motherboard to see if it's possible to upgrade the processor.
It's a proprietary motherboard and may or may not be able to take a processor upgrade. If it does, however, I'd be looking at spending the remainder of your $100 (after the RAM upgrade) on a processor upgrade; the best e-bay deal you can find (and probably nothing over $60).
Unfortunately, without knowing exactly what motherboard is installed, it's impossible to even say what RAM or processors might be compatible.
Please verify that the system I linked above is in fact the system you have. If it is, you're also limited to single slot width graphic cards given the system's BTX form factor.
-Wolf sends