Totally depends on the games you play and resolution you play at. GPU intensive games, high resolution, fsaa ...SLI will help the most. But in a CPU limited Flight Sim or RTS game, the CPU would help more. Also consider games like Oblivion in which some areas (outdoor foliage) are very GPU bound and other areas (in towns, lots of NPC's) are very cpu bound.
I'm with these guys wondering why you want to upgrade? If I had to choose for general gaming I'd rather have the SLI route, especilly on a 22" or 24" LCD. But really, then you'd want a faster CPU to best take advantage of the dual cards. So if both upgrades are out of the question, you may want to just reduce settings a bit, keep gaming, and put that $300 toward a rainy day or your next build.