The difference between the two is that one of them is that the more expensive one comes with 4GB of GDDR5 VRAM and the one for $499 comes with 2GB GDDR5. The one with 4GB is better for gaming at very high resolutions, it usually shows benefits at resolutions of 2560x1600 and in multi monitor setups. Under resolutions of 1920x1200 and below 2gb is usually enough. Also if you play games with a lot of mods (Skyrim being a good example) having more VRAM can be helpful.
If you are looking for a gtx680 I would get one with a non-reference cooler, like a Gigabyte Windforce, Asus DirectCu, or MSI twin frozr/lightning cards.
Also you could save some $$$(~80-110) and go with a gtx670 as there is only a slight performance difference between a gtx680 and a gtx670. In fact a gtx670 can match a gtx680 when overclocked, and is overall a better value.
So I would recommend a non reference gtx670 and overclocking it to match a gtx680. A non reference card will run cooler, and should allow you to overclock higher.
There's a huge difference between ATI then and now. AMD Radeon 7970 will do you good and save you tons of money. You can get a Sapphire HD 7970 for 394.99 at www.ncix.com and that's almost 100 less. so you get 2 and save 200 and they OC like crazy. Mines are running at 1100/1600 and are super quite. Cheers
Just throwing out there, the 680 is a bad buy. It's only 5% faster than a 670, but it costs 25-30% more.
You'll get basically the same performance out of two 670s, for $200 less.
Also, just asking, what monitor setup are you using? If it's a triple monitor setup, you want two 4GB 670s. If it's a single monitor setup, look at some benchmarks; more than likely a single 2GB 670 is more than enough for your needs.