lol.. past few months? sounds more like few hours
anywyas... it does not look like you can use SLI because you only have ONE pci-express x16 slot, and you need 2 of them to run SLI. also, with SLI, you can only use the same cards.. you can't use a weaker card with a strong card, so if you have a 7900gt, you'd need another 7900gt.... BUT.. your motherboard doesn't support SLI anyways, so you can't do that. if you wanted SLI really bad, you'd need to get a new motherboard as well.. one that supports SLI... and thats a pain because it might not fit in your dell case and might not be compatible with a lot of the dell components like PSU or w/e... and you'll also have to do a fresh install of windows... (and if you have a product recovery disk, it might not work..)
oh and no, you can not upgrade video memory like that.. you'd just need to get a whole new video card...
but your computer seems very capable... what aren't you able to do with just one 7900gt?