We have had extensive experience with running 4 monitors on the xw6400, xw6600 (and their more powerful brethren the xw8400 and xw8600). Also on the Z600 and Z620 workstations.
The power capabilities of the video PCIe x16 video slots has gone up over time. For example, the xw6400 top ("primary") video slot can feed 52 watts up to the inserted card, while the lower PCIe x16 slot can only feed 25 watts up. The xw6600 can do more, and the Z620 can do even more in terms of the video slot performance.
What we developed for our medical imaging needs was to put the far left and far right color monitors on the top card, which is a nVidia FX1800, and put the central grayscale left and right 3MP or 5MP monitors on the bottom card. That bottom card is the more powerful FX3700 and its gets 25W from the slot and up to 75W from the 6-wire PCIe x16 supplemental video power cord that is zip tied currently up behind your power supply. Thus, more than enough.
This way both cards get enough power, all have been running perfectly for years now, and those cards can be bought at a very reasonable price off eBay, used. The FX1800 has a lower DVI port (primary port, and thus the one that BIOS image shows up from if it is in the primary upper video PCIe slot). It then has two adjacent additional DP ports... use the one next to the DVI port. You can buy a DP to DVI adapter off eBay.... specifically get the HP one from BizLink because it's DP end fits properly under a metal case ridge right above the top backplane opening where that card has to go. Most of the other adapters won't fit in that tight space, but the BizLink one will. The FX3700 has two DVI ports.... again, use the lower for the left of the two center monitors.... for us those are the high-megapixel grayscale ones.
If you are stuck with 4 VGA monitors you actually can get DVI to VGA video cables, and also can get a DP to VGA adapter from BizLink off eBay. Go digital if you can....
This setup will be fast, rock solid, and proven.
