Using an SSD is clearly noticeable compared to the normal laptop hard drive. Login is faster, normal applications launch instantly (the icons in the dock almost don't bounce).

After enough RAM is the best upgrade you could get. Well, technically it's also the only other upgrade you could get to your laptop, but it should make it snappy enough to delay a new laptop purchase for a while.

BTW, what would the £1650 be for ? 4GB of RAM is probably £100 max, an Intel 160GB SSD another £400 max.