Two things your CPU is low on GHz (can be overclocked) and the 7600 Nvidia cards did not age well specially dealing with shaders. Perhaps upgrading to a GTS250, HD4770 or HD4850 for about $90-100 will allow you to play it at acceptable level.
I have a 4 year old machine (specs below) and I'm wondering if I can upgrade my graphics card to buy some time before needing a new mobo/cpu.
I just got the new Battlefield Bad Company 2, and I get some nasty screen lag/hanging.
Was just looking at an XFX Radeon 5770 on Newegg for $159.
Asus A8N-SLI Deluxe
AMD X2 3800+
Windows XP Pro 32/Windows 7 64 bit dual boot
MSI nVidia GTI 7600
Compared to my system - AMD FX-60 dual core, A8N-SLI Deluxe, 2GB of CL2 Corsair, 8800 GTS 512 and Windows XP, my rig barely runs this game. I have to turn everything down in order to get moderately playable FPS. If I turn things up any more than that, its like watching a flip-page picture book.
don't bother with a gpu before you get a new cpu. I have the 4800+ of that same series and i can barely keep my neck above 30 fps normally, let alone with tanks firing and things getting destroyed left and right