I have one of each. If you were planning on doing some gaming, then from a performance perspective, the GeForce2 GTS will whup the A7N266-VM's onboard video by a factor of of two to three, and the SiS735 platform used in the K7S5A is actually a little faster in most stuff than the nForce 220D platform used in the A7N266-VM.
The onboard sound of my K7S5A is pretty low in quality, while the A7N266-VM is on the opposite end of the spectrum, with nVidia's Soundstorm (yes, it does have it), Dolby Digital encoding and 5.1 support, including an S/PDIF optical + RCA outlet plate in the retail-boxed version. If you were going to use the GF2GTS regardless of which board you got, the A7N266-VM would be my first pick for the good sound. It also comes with an auxiliary outlet plate with two extra USB outlets. My own K7S5A has been fine but there were a fair number of people griping that they couldn't make them work, and Asus is offering a 3-year warranty, so that may be another good reason to spend a little more and pair up the GF2GTS with the A7N266-VM.