I agree with the previous message. You could buy a new processor fairly cheaply, but you could also get a good deal on a new board/chip combo or barebones system for a little more. Start browsing the Hot Deals thread (if you don't already) and you may find your answer relatively easily. About 6 weeks or so ago, there was a post for a barebones system (mobo, CPU, case w/ 300W psu) for $199 from Frys. Oh...should point out this was an XP 1700+ system. That was pretty good deal, but it looks like processor prices are dropping further, so you can probably find a faster machine for the same price, or the same speeds for less. On the flip side, RAM seems to be increasing in price, so you probably want to upgrade to a system that will take your existing RAM to keep costs down. I believe you can find mobos that takeboth DDr and PC133 RAM (much like older systems had slots for 30-pin & 72-pin SIMMs, 72-Pin & 168-Pin, etc.)
You can also check Pricewatch t use as a gage, but from what I've seen, many of the deals in Hot Deals are better than the prices displayed on Pricewatch. You did not specify your current setup, so it's hard to say, but it may be beneficial for you to buy a bare-bones package that includes at least a 300W PSU. The barebones system I bought last year (750 Duron, Pc-Chips M810LMR mobo, 256MB PC133, & 300w PSU/case for $199, including shipping) has worked very well for me. It gave me a quick upgrade to my RAM, got me the upgraded PSU i wanted/needed, and basically had everything I needed in 1 quick purchase. Until I found that deal, I was originally looking for the CPU/board and the case as 2 two purchases. I found this package, compared specs, and found the one I got to be cheaper and included the RAM as a bonus. Set your budget, then look to see what you can get for it using the Hot Deals forum. You'll probably be surprised.