Should I buy all new memory or combine with old? I tried googling for answers on this, but couldn't actually find any decent information. I have a mid 2010 Macbook Pro, which according to Apple supports up to 8GB of ram. I already have a single 4GB stick installed, and from what Google tells me about the identifiers from System Information, it's a Corsair (I haven't actually opened the machine to try and verify.)

I need more memory, as I'm constantly bumping up against the limit and the system starts swapping instead. Now my question is, what configuration will yield me the best performance? Two 4GB sticks? One 8GB? If I go for a configuration with two 4GB sticks, is it better to get two new ones or just add a single 4GB (presumably same make and model) to the one I already have?
The 8GB would be via two 4GB modules.

As far as replacing vs adding, the DIMM slots are dual channel so you'll just want to make sure they match. Look again in System Information at the speed column. It will likely say "1066 MHz". In that case, when shopping around, just make sure you're looking for a 1066MHz DDR3 4GB 204-Pin module. It doesn't have to be the same brand.