I did something similar... got a new AXP 1700+ (o/c to 2GHz or 2400+) with Epox 8RDA and DDR SDRAM, and put ye olde 800MHz Duron with its motherboard and RAM in the case that formerly held my 500MHz Celeron. Now the Celeron processor, along with its motherboard, RAM, and PSU are sitting caseless and diskless next to me on the desk, begging to have more money spent on them so they can turn into a working system. 😉
Unfortunately, I recently found a good deal on a laptop (some Compaq unit in decent condition, with 700MHz cpu and 128MB RAM - not sure on HDD specs - for US$450!), so if I do get enough money to spend some on computer stuff, that's where it will be going. 😱