Upgrading FireGl V3400, Budget <= $600

I was recently given a Dell Precision 490 with dual Xeon 5060 dual cores. The workstation came with a FireGl V3400, and I'm looking to upgrade to a better graphics card.

I am running Windows 7 Ultimate x64.

I will be using Pro/E (now called Creo Elements Pro) to do a lot of rapid prototyping, some rendering, and some simulations using COMSOL 4.0. tc. I will also be using Cadence OrCAD for simulations, designs, routing, and circuit solving. I plan on playing a few light games like Osmos and Plants vs. Zombies in addition to heavier games like Starcraft 2 and Diablo 3. Finally, I will be watching movies, handling large files, and listening to plenty of music.
Right now, this system has a 1280x1024 monitor, but it might have two of these monitors soon.

Up until now, I have used a laptop sporting an intel T7200 dual core with 2G DDR2 700MHz, and a wonderful Quadro FX 3500M (on a 1920x1200 screen). I've done most of the above on this machine, and it has served me very well. This is just so you know what I'm used to. I suppose the Xeons are an upgrade, especially with the 4G of ram and a much faster FSB.

One final note... the system has a 650W psu... I don't know if this limits anything.

*EDIT* BUDGET is preferably less than $600

Current system:
Dual Intel Xeon 5060 3.2 GHz dual core processors
Intel 5000X chipset
4 GB DDR2 FB-DIMM ECC memory
ATI FireGL V3400 (what I think should change.)
Standard 650W psu
80GB SATA x2 (RAID)

Motherboard Features:
One (1) PCI Express x16 slot (occupied)
Two (2) PCI Express x8 slots wired as x4
Two (2) 64-bit/100 MHz PCI-X slots
One (1) 32-bit/33 MHz PCI slot
Five (5) SATA 3.0 Gb/s ports with RAID 0, 1 and 5
Eight (8) 240-pin DIMM sockets (four occupied)

Thanks so much; I can't wait to read the responses.