Doesn't matter, they will all be within a couple bucks of each other. Look for free shipping then flip a coin
Other than RAM the next best thing to do would be to throw in an SSD.... but I don't know if it would be worth it in your budget range etc.
You could get a crucial M4 128GB SSD for about $120 or a 256GB for $200...
I don't know if your laptop has enough space for an SSD and an HDD... so if I were to get an SSD I would make sure I got one big enough for my needs or that I could still make use of the HDD as a storage drive.
Yeah, I haven't actually opened it up yet so I don't know if there's space for two drives. I'll be able to do that soon, though.
Is there a possibility of upgrading the GPU or would I need to replace the PSU to do that?
Laptop GPU's usually aren't upgradeable; even if they are, the CPU would bottleneck it.
I'm pretty sure the 9000 series has a place for 2 drives, one on each side underneath.