I recommend a 680i board and an Intel Core 2 Duo proc. You can get both on the cheap and are still not "low end". What board/proc do you have now? Is your memory DDR2? Is your video card AGP? DDR1 and AGP are tougher to find support for in newer boards. Also, what price are you looking to stay under?
The cheapest proc (Socket 775) is the
Celeron D 360 for about US$60. If you can go higher, I recommend at least a
Core 2 Duo E6300 for roughly US$175 (though an E6600 or E6700 will give you more life down the road).
For a board I recommend an
EVGA 650i for under $100, though as I said, the 680i is a stellar chipset for stability and performance.