just upgrade to 8G of RAM and you should be fine 5boxing. you may get a little lag in cities and high population areas but if you get 4G more RAM you should have no problem 5boxing.

if your motherboard won't support 4G more RAM then you would be best off either replacing it or getting another box for either a slave or your main (i would go with main but that's just me)

hope this helps some ^_^

~YYF