If you want something faster than the 5700U get the 9600xt.
Actually, in the tests I saw at Tom's Hardware Guide, those cards are practically identical in performance, but I haven't compared them myself.
Also, I'd like to add my observation that the 9600XT I tested was only slightly better than my current GF4 TI4200 in the default Seattle flight in FS9. The 9600XT was definitely faster in antialiasing and anisotropic filtering, (heavy cloud, distant textures), that gave it the edge. For raw speed, the old GF4 TI's are still up there with respectable performance.

Mainboard: Asus P5K-Premium, CPU=Intel E6850 @ x8x450fsb 3.6ghz, RAM: 4gb PC8500 Team Dark, Video: NV8800GT, HDD: 2x1Tb Samsung F3 RAID-0 + 1Tb F3, PSU: Antec 550 Basiq, OS: Win7x64, Display: 24&