A 5970 is the only GPU capable of playing of playing crysis well at 1920x1200 resolution. You need a new motherboard and high end quad core CPU to take advantage of that cards massive graphics power. An i5 750 or Phenom II 955 system would make an awesome gaming machine. http://www.tomshardware.com/reviews/radeon-hd-5970,2474...
First thing you need to do is verify that processor is even compatible with your motherboard. If you're not sure, list your motherboard manufacturer and model. Next would be to increase the amount of RAM to at least 2GB (3 or 4 GB would be preferable).
First off, the E5400 is not compatible with that motherboard.
Second, that motherboard does not support ANY processor that can handle a high-end GPU.
Third, your lag was probably caused by lack of memory more than anything else.
A P4 630 and a Radeon 5970, with only 1GB of meory? That's priceless . You would have been way better off with a new AMD Phenom II, RAM, crossfire board, and a 4850 to crossfire with your 4830 for probably a little less money. Anyway...........
Your current board can only support the older P4s like the one you currently have. It will not support C2 based Pentiums like the 5400.
If you got someone to buy your 5970 use the money for new RAM, CPU, and Motherboard. You may even have enough left for a 4830 or 4850 to crossfire with your current 4830.