Wirelessly posted (This odd thing that I hold: Mozilla/5.0 (iPhone; U; CPU iPhone OS 3_1_3 like Mac OS X; en-us) AppleWebKit/528.18 (KHTML, like Gecko) Version/4.0 Mobile/7E18 Safari/528.16)
If it's just general snappiness you need then I'd seriously think about a SSD. Your laptop is 3 years old like mine, so it won't take full advantage of the fastest SSD, so you could buy an older cheaper SSD.
How much space do you really need?
An advantage of a SSD is that you will get several years extra life out of your laptop, more than with an XT. Your laptop is still one of the fastest around, so with an SSD there's little reason to upgrade.
How much RAM do you have? It might be better to upgrade that if you only have around 1GB RAM total.