Trouble With 4 Monitors

[The Dude]

I posted awhile back asking advice on where to get the hardware to run 4 monitors. Well, I got the stuff in the mail and got it all set up. Here's what I'm running:

4 Samsung 213Ts (21.3 in flat-panel monitors)
Xentera GT 4 (256 MB DDR video card, PCI, 4 DVI outputs)

This is the computer I already had, that I'm currently running this on:

HP Media Center 1070
- 520 Pentium 4
- 512 RAM

While by no means am I computer illiterate, I'm in a bit over my head with this system. Since I installed everything, my computer is running extremely slowly. CPU usage hovers around 15% with no applications running, and jumps to about 40% when I run poker clients, Poker Tracker, PlayerView, Firefox, and iTunes. But when I try and move windows or load new windows, the computer drags tremendously.

The video card claims as system requirements:
-Windows 2000 or XP compatible (Pentium 4 recommended)
-128 MB RAM, 256 recommended.

I obviously meet these requirements, so I'm not sure why my computer would be running so slowly. If I simply buy more RAM, is this likely to fix the problem? Should I just go ahead and dump the machine I've got in lieu of something more powerful all around? Is the problem possibly caused by something on my system that I can fix? (I looked at all the processes running under Task Manager, but I don't konw enough to recognize what I can and cannot delete.)

I just got the system set up very late last night, and by the time I woke up today the customer support numbers I have were closed.