So you got 5040x1050 working after all ? ;)
Wings of Prey is a pretty graphics-intensive game altogether, so obviously running it at multi-monitor resolutions
will have a performance cost. Getting a second 768 MB card would help, but not that much - 1, 2 or even 3 cards would still be hindered by the low quantity of VRAM. If you like full-on FX and extreme resolutions, you should get cards with at least 1 GB of VRAM.
Read Paradigm Shifter's excellent article on the subject:
NVIDIA GTX460 in Surround: 1GB vs. 2GB - Featured Review
Now, going SLI (preferably with 2-GB 460s :) ) would let you get rid of TripleHead2Go and use Nvidia Surround instead. Main benefits: no custom resolutions & refresh rates to manually set + full support of DX10/11 at 5040x1050. Drawback: next time you wish to upgrade your graphics system, you'll have to buy two cards to be able to game on three monitors.