Hi mate,
I've bought two Dell laptops in the past (one gaming one for me and a casual/work one for the wife) and both are now on their way out. Performance has just degraded so much, so quickly that I wouldn't buy from Dell again. Tech support wasn't bad but they charge an arm and a leg. I am never going to buy another laptop again - unless it's a Macbook Pro. I wouldn't buy another one for gaming anyway. My Dell laptop ran as hot as hell even on day 1 when I was gaming.
Never again.
Before going out and buying a GPU, even a £40 one, you'd need to really check to see if it's compatible or not. I'd say your current CPU is going to really hold back your GPU unless you get one that's like 3 years old, and even then it probably won't be that much better than the GPU you currently have.
I'd definitely recommend building your own PC; it's cheap as chips when compared to purchased a retail PC. If you live near my neck of the woods I'd even be happy to lend a hand. I'm constantly upgrading the flip out of my current PC (about to order another 4GB of RAM and possibly upgrade the CPU to a hexa-core) and it kinda becomes a hobbie after a while... a ruddy expensive one, mind!
Also, yeah, you can upgrade your memory on al laptop but your warranty will be instantly voided. The second you break the warranty seals and catch a glimpse of your laptop's innards then Dell has the right to say "Lol! On your bike son!" if ever you need support. Also, I've never tinkered with the insides of a laptop before so I wouldn't want to offer any advice on the matter - I know I'd old brick the thing!