Kekkerer is correct. While there would be a performance gain from getting a new GPU, there is more to be gained by upgrading your CPU.

Was financial reasons the reason for...cheaping out on your multiboxing rig?

Go big/absurd or go home! (For absurd I'd throw in as many of those 240GT cards in your motherboard as you can, assign different slaves to them and then get some cheap monitors and give each slave a monitor)