I would go with crucial, or pull one stick and check the brand and actual speed on the label. Upgrading ram means simply adding more; no point in paying for performance ram as the dell has no ram adjustments in the bios. You can use up to 2 gb per slot for a total of 8 gb. Mixing ram brands or speeds may not work, so checking the current label may save you from having to return it.
Current Ram is Samsung 667 MHz. I want to upgrade to higher MHz if that can add to performance. Mostly I use computer for gaming so increase in RAM speed might help. Also I will not mix Ram with mixed speeds, as alll will run at speed of lowest RAM inserted.
If the board says it will run 800, then pc6400 should work. But don't expect that much difference performance wise. Don't pay more for it over pc5300. I still recommend you match the brand and speed, unless you buy it locally so you can exchange it. Frys has a 15 day return policy with no restocking fees, but their stores are mostly in the west southwest and atlanta areas. Microcenter may have a similar return policy, but check with them first.