Well, when are you going to buy?
If right now, the GF4 is not available. That leaves the Radeon 8500 and the GF3Ti500.
What games do you play? The Radeon's and GF3's respective architectures each lend themselves to some games better than others. So, if you could post which games you are/want to play, we can tell you which card would be best suited to you. Also, do you need GOOD 2D? Dual-monitor ssupport? TV-out? What about price; how much are you prepared to spend? All these factors can come into play.
If it helps any, I currently have a Radeon 8500 (retail) in my primary workstation, and a GF3Ti200 in my LAN box.
Give us a little more info, and we can help you out much more easily.