GTS 240, 1024MB (concensus is this is just a supercharged GT 9800)
I'm suspicious of OEM-only products like the 240. Why isn't this released to consumers? Well, maybe it's only there so that Dell can boast about having “1GB VRAM!!”. This amount of memory probably isn't justified by the specs of the GPU itself; 1GB is big even on a full 250.
You say you're not doing high-end gaming; if that's the case both those cards are hugely overpowered and will generate far more heat and noise than you need. Don't buy a big butch power-sucking 3D monster unless you're really going to use it.
ATI are generally considered to be winning the mid-end at the moment. Take a look at eg. the 4650, which is widely available with passive cooling (zero noise). It's not the polygon-masher the 4870 is, but it'll cope with any of today's games.
Hell, “Starcraft and video editing” will run just fine on the poxiest of current integrated chipsets.