well first if you wan't an instant boost you need to buy a SSD.
second you can upgrade the ram to 6 or 8gb, that is if you can find 4gb ddr2 modules (it exist, i have one).
third you may want another HDD for extra space, you need to buy a HDD Caddy to replace your optical drive.
Fourth, you can actually upgrade the CPU to Quad core if the model supports it. But this kinda experimental, so you've been warned.
Well that's all you can do.
That laptop is too old.