The 8800GT for pre-2008 Mac Pro's is only a rumor at this point, and as has been demonstrated, performs well in games but very poorly for Pro Apps ( thank you Nvidia).
If you balk at the notion of spending a high £270 for another X1900, your best option is £90 for the Radeon 2600XT, which works in pre-2008 Macs. It's slower in games but performs at parity with the X1900 for pro-apps.
Of course, if you haven't done so already, disassemble your X1900, clean out the dust in the intake and fan assembly, and apply some Arctic Silver thermal paste to the GPU/Heatsink interface. I had alot of graphics lock-ups with my X1900 that went away after I performed these steps.