I have two 19" 1440x900 Monitors running off a 3850. I've had no troubles what so ever. I do alot of gamin too, TF2, CSS, and others. I"m curious as to what you're doing that your card is not enough. I've always understood SLi/Crossfire is unnecessary unless you're rocking out the most powerful card already or have higher resolutions.
Yea, at that resolution the gains from multi-GPU are slim to none, there is just not enough display real estate to stress a single card. Your 4850 will be more than enough until you upgrade to a larger monitor.
The general rule as I understand it is 1600x1200 or 1680x1050 and up to see significant gains from multi-GPU setups.
It might be worth it, if you really must play crysis at the highest settings (including AA). Personally, I see no point in AA beyond 8x (at 1280X1024 and up) , and 4x leaves me quit happy. Personally, I would never consider SLI or Xfire due to the diminished gains, increased noise and heat and price, and generally lack of need - at standard resolutions like yours.