Need 8GB RAM for Good Price

I put together a cheap build for my work computer (home office) with 4GB of RAM which I thought would be enough, but as I used a APU and it uses about 1GB of RAM then another 2GB of RAM at idle while leaving about 1GB left for other tasks.

I only have 2 slots for RAM so I'd need either 1x8GB or 2x4GB of DDR3 RAM (240 pin).

I don't have a preference but I like G.Skill and Corsair, I'm not too concerned about MHz or CL as long as its not low MHz and high CL combined.

Something on eBay would be a bonus as I am expecting my $50 voucher but its not necessary as I also need to buy car stuff there and can use my voucher on that instead.

Thanks for any help.