Get a gig of memory, a decent processor and anything better than a 6600 GT will power your extra characters at 1280x1024 no problems. It's not the resolution thats the killer its the bells and whistles combined with the high resolution. I crank all of the settings save for terrain distance down on the Zins for maximum FPS - I mainly look at Xzin's screen anyway - which is a far beefier system and can handle max settings with nearly capped FPS. The biggest cause of latency is lack of memory and possibly a fragmented hard drive. Short of a DRAM based hard drive, defrag every so often and make sure the CPU and GPU is reasonably fast. Make sure to make every system identical if at all possible. It does not have to cost a ton to roll 4 more accounts but make sure you think about input and how you plan to set that up. Vetra makes the best keyboard broadcasters but they are not cheap - $199 a pop and you need 2. Go with monoprice.com for PS/2 cables - far cheaper there than anywhere else.

All told, you can add 4 extra computers for around $2k not counting the monitors or extra hardware - just the CPUs and have a rock solid platform that will not be obsolete come next expansion and will manage 50+ fps constantly.