Get the FX53. Your applications will not benefit from a dual system. If you were doing content creation such as animation, graphics or video, the story might be different. It would also depend on the apps, even then.
A little myth busting here... Opterons are not really cheaper than Xeons at the desktop level in the 2.4-3.06 GHz range. When you factor in the mobo, like for like Xeons can be cheaper. If you got 2.4 M0 Xeons, a PC-DL mobo, and did some mods (involves solder and removing a pin or two 😉 ), you can get to about 3.4-3.5GHz.
Examples - a 244 is about $355 and 246s are in the $400 range. Tyan has finally come down to about $230 on the K8W dual.
When you get to the server level where you are using the MP Xeons, then Opterons do get cheaper (sometimes, lots cheaper). Once you get past dual processors, Opteron also starts spanking butt with the bandwidth provided by Hyper-Transport.
As for quad processors, XP Pro supports 1-2 processors. You need W2K or W2K3 Server if you want to run more processors. Add another $600 to the price 😉