I know you are on a limited budget, and can't afford a new computer. Based on the specs of your current machine, I'd seriously question whether you should be spending anything on it.
Assuming that this is your motherboard
http://www.nec-computers.co.uk/suppo...lt&source=1409, you are limited to 2GB of RAM, in 2 x 1 GB units. That means you'll have to ditch the RAM you have to upgrade. More RAM will provide a mild tweak in speed, but only when you really do start to manipulate some bigger things. However at that point the limited potency of the CPU (Celeron processor) will remain a significant bottleneck, even with a graphics card added. And the point is that, because the PC base spec is now quite old, you're effectively paying over the odds to buy older compatible kit that is sitting on a shelf somewhere.