If you upgrade the memory to 3GB you can easily run 5 accounts at low settings.

I have the same video card, a slower processor (x2 but only 2.2GHz), and 3GB of memory, and I 5 box just fine using 5 separate wow folders, and the lowest res/settings on the 4 clones. The primary I up the res to 1024x768 and leave sound on and its fine.

Another bottleneck when you run 5 wow accounts is the hard drive = having a second hard drive is very good, put 3 on one and 2 on the other and theres a huge lag decrease.