Well I did the build this weekend, the only difference was I decided to wait on the GX2's after reading an article on tomshardware.com about whether cpu or graphics card upgardes give better bang for the buck...seems that at 22" res the difference between a 9800GTX SLI and GX2 Quad SLI is non-existant so decided to see how the 7800GTX's I had went first.

Came to 2 conclussions:

1. Putting one monitor on one card and the other on the other with the cards not in SLI gave horrendous framerates...single digit fps on all accounts.
2. Putting two monitors on one card with SLI disabled gave acceptable framerate but lower than I was getting on a single 7800 with 4Gb of ram under XP32 (eg all screens getting ~15fps under Vista64, main getting ~30 and background at around 15 under XP32)

I'm not sure if taking the SLI bridge off the 2 cards will make a difference, but at this stage I'm still undecided which way to go for graphics card upgrade. Going with 2 GX2's will provide no real benefit to other games over 2 9800GTX's when in SLI, but I have a feeling a GX2 will provide better framerates than a GTX while multiboxing on a single card.