Quick question on a graphics card.

[MiRai]

Sooo will they do a decent job at what im aiming to do?

I don't really know. I don't own either one of those nVidia cards, but the GTX 660 looks to be the stronger choice of the two. Honestly, if you can deal with lower-ish video settings (possibly even use DX9) then you should be able to run 5 clients without issues on most mid-range to low-end video cards (assuming the CPU is up to the task and you've got enough free RAM).

Even if the GTX 660 isn't able to display the eye candy you want at the FPS you want, you can't upgrade because the next step up is a GTX 670 and that requires a 500W PSU at the very minimum. So, you're kinda stuck.

Assuming you don't want to run your main client all of your clients at Ultra with multisampling @ a solid 60 FPS, I'd say you should be okay.

EDIT: Apologies, I hastily wrote out the last statement because my delicious pizza was ready and have corrected it.