If you're running sdram it might not be a big boost to get more cpu speed now. If money is an issue than you can pick up a brand new 745 Ultra for $35 shipped on pricewatch, and pick up a generic cheapo 512MB stick of PC3200 for $58. $93 for an easy 333MHz and your existing 1600MHz cpu will probably give you more performance than spending close to that much on a 2500+ and overclocking it on an sdram motherboard.
If you have a few more bucks to spend, then get an Nforce2 board for about $20 more, but then memory quality will be an issue and you will have to spend more money to run 200MHz fsb speeds.
If you have a few more bucks to spend, then get an Nforce2 board for about $20 more, but then memory quality will be an issue and you will have to spend more money to run 200MHz fsb speeds.