Re: 5700 or 9100
5700 i'd say. The 9100 is a very old ATI chip (based on 8500 i think). It was good a few years ago but now it's just outdated.
Why don't you save a bit more up and get a 9800 PRO.
You also could get an used 9700 PRO. It's better than both and should be available on ebay for a reasonable price.
Intel Xeon X3350 @3.5Ghz
DFI Lanparty DK X38 T2R
2x 2048mb Apogee GT
Gigabyte 8800 GTS G92 (800/1944/2050)