I haven't been able to find charts with test results using a dual core system (at least closer to my speeds) and an 8 series video card, they all seem to use a quadcore CPU. Any links?
Also, is it worth it to buy just one card or do you need to be running two in order to get good frame rates in the newer games?
Keep in mind that on most games (that don't support dual or quad core technologies), you won't really see an improvement over having just one core. Yeah, XP/Vista can attempt to offload some of their background stuff onto other cores, but for games that don't support anything beyond a single core, your only difference will be raw processor speed.
And stick with one video card. By the time you might be interested in going SLI, odds are there will be a brand new card out that beats the older one by more than 2x.