I would focus your efforts on Video card, memory, motherboard (in that order) before a new processor. The 2400+ is a good processor when compared to a 2800+.
I am a die-hard Intel fan. I have run lots of AMD chips, but I always end up back with intel.
The difference between a 2400+ and a 2800+ are worth the price. Since the performance really isn't that much of an improvement.
Tomshardware Guide tells me that in 2800+ vs. 2400+ is 307.2 vs. 261.5 for OpenGL and 15427 vs. 13257 Direct3D, respectfully.
Here is a link to the article.
I think a lot of these people here will agree with me as well.
Happy upgrading! :-)