I have a WinXP Pro box, a Radeon HD 4650 512MB PCIExpress card (with latest drivers) and 2 19" lcd monitors. 1 LCD monitor (westinghouse) is VGA, the other (Gateway) is DVI I run a scrneesaver from ...
I have a workstation in the bedroom, 18" LCD attached to it as well as a 42" plasma. XBMC stays full screened on the 42" all the time. The setup works great, sans one tiny little thing, screen savers ...