DDR 667 vs DDR 800
You've got quite a long way to go buddy...go read some overclocking guides before you buy anything
1. I hope you're not planning to overclock a Dell PC, because that'll be very difficult and not worth the purchase if you're actually interested in overclocking.
2. Technically, there are a few programs that can overclock from within Windows, but real overclocking is done in the bios. More than likely your Dell PC will have all the overclocking options removed from the bios so this wouldn't even be an option for you. Don't be scared to go into the bios, there's very little you can do in there that could fry your system.
Now if you're just trying to buy the RAM from Dell, I'd start off looking elsewhere because you'll probably find cheaper, higher quality RAM than what Dell has to offer.
And go with DDR2-800. DDR2-667 won't give you much more room for overclocking than what what you're already running (probably DDR2-533)
I don't know if you'll understand any of this, but a simple equation for determining your overclock is to take the multiplier of your chip and multiply it by the FSB (Front Side Bus) of the processor.
In the case of the 6320, it has a 1066 FSB (4 x 266) and a 7x multiplier.
With DDR (Double Data Rate) memory, the rated speed is actually twice that of the front side bus. For example, DDR2-533 would be 2 x 266mhz, DDR2-667 is 2 x 333mhz, and DDR2-800 is 2 x 400mhz.
With DDR2-667, assuming it won't overclock any higher than DDR2-667 speeds, your maxium overclock would be 2.3GHZ (7 x 333). This would only be a 500mhz increase from stock speeds, which IMO isn't worth the savings between DDR2-667 and DDR2-800.
With DDR2-800, assuming it won't overclock any higher than DDR2-800 speeds, your maxium overclock would be 2.8GHZ (7 x 400). This would be a 1GHZ overclock from stock speeds, which is a more worthwhile overclock IMO.