Yes, the 880GT is a bit old. At 3.2GHz, maybe a GTX285 (16,462) or HD4890 (17,255)?
The HD4850 is a little bit better, but certainly a more modern replacement for the 8800GT. Tom's 3DMark06 benchmarks show the following:
My own benchmarks PII X4 940 @ 3.5GHz indicate 13,208 for a single HD4850 and 18,494 for Crossfire.
If your 8800GT went belly-up today (ie died) then the HD4850 would be a good replacement.