|Now that video drivers are allowing multi-gpu setups to actually scale well, sli/xfire is nowhere near as risky an investment as it was a couple of years ago.|
Radeon HD 4850 crossfire setups were (and still are) extremely popular in Midrange AMD land. The performance and price of such a setup beats a single HD4890.
I'm betting that when the HD 5850 becomes available, running two of them in xfire will be just as popular. Two 5850s should easily outperform a single 5870.
However, with a 20 inch monitor, you may not need more than a single 5850. It'd be a good idea to buy one first (when they're available), and add another if/when you want the extra performance.