I've been doing fine on a small curved computer desk (purchased at Eurway) with 3 21" monitors on it - if these can fit, almost anything else can fit.
To be honest, I would be very tempted to get 3 of the nice looking Dell LCDs, 19" with the thin silver bezel - and those would fit anywhere...
The Parhelia has the restriction, the 256Mb version has the same restriction - I would bet that it's a restriction on the bandwidth of the card's secondary channel, since the primary channel feeds one display and the secondary channel gets split - it's effectively running 2560x1024x32bpp, which is a lot of information to be shuttling.
Also, if you are tempted to stay dual head, I would recommend just keeping your G400 - I have used both GeForce 3 and 9700 Pro cards, while they seem to be reasonably stable, I would not consider them in the same league as the G400 drivers - those have been rock solid in 2D. Now, 3D, the roles completely change - I wouldn't trust my Matrox nearly as much as I would the other cards - different markets, different focus for the drivers.
Oh, last time I checked, Dell has a sale on the LCDs - 19" for around $699, IIRC, they might still have a discount going on - go to their small business site, not personal site.