Keygrip,
As has been mentioned an SLi set up will only need to be used on the very most demanding games and only then when you want to play with the prettiest settings.
You are better off paying more for a single more powerful GPU rather than having two slower graphics cards in SLi.
My current setup is on a single older (2 years+) 8800gts, which runs quite happily a 30" monitor at 2560x1600 and an a 20" at 1600x1200 and it easily copes with Everquest II on the larger screen with a browser on the smaller - both connected by DVI.
Of the current decent cards out there the GTX 275 is around £175 and is more than ample to deliver your needs effectively - or another cheaper card which is good is the ATI 4870 which incidently can later be paired up using Crossfire (ATI's equivalent of SLI).
Personally I would save on the motherboard expense and go for a more expensive graphics card.