Flip through the benchmarks. It's pretty sad that the 630 loses a couple of gaming benchmarks, but is otherwise faster in other gaming benchmarks and applications.
If your current motherboard is overclockable, why not just overclock it? There are various nforce 3 and nforce 4 overclocking utilities even if your board has no overclocking options. You may or may not be able to overclock the Dell (some boards support 1066MHz FSB, and 3GHz to 4GHz is pretty iffy).