For anyone who's interested / searching for an answer in the future - it works! With a cheapo Nvidia (about £50 when I got it) and a cheapo DH2G (£30 from eBay), I'm running three screens with OpenGL / Wine D3d HAL etc.
So I'm running:
1x Nvidia 7600GT
Output #1: DVI @ 1280x768
Output #2: Matrox DualHead2Go (Analogue)
Matrox Output #1: 15" Analogue TFT @ 1024x768
Matrox Output #2: 15" Analogue TFT @ 1024x768
Under Ubuntu 8.04 and using nvidia-settings, it works as either
a) Two separate X-screens
b) A xinerama-merged screen at 3328x768 for OpenGL etc.
c) An Nvidia Twinview screen at the same, also for OpenGL etc.
Works with Enemy Territory: Quake Wars, OpenArena, Steam (via Wine), Deus Ex (again via Wine), etc. etc.
Amazingly, it's playable in ETQW, even at this res. and with a fairly mediocre graphics card. Steam-games (CS:S / HL2 Ep2) are low FPS (~30, sometimes a bit below) but playable.
If anyone's looking for any help setting it up etc. PM / reply and I'll attach any xorg.conf etc.s that anyone needs.