You don't have to spend more than £105 for a significant improvement, but don't go for anything less than a 8800GT.
PNY 8800GT 512Mb click here for £105 (free delivery click here). If you can find a cheaper 8800GT elsewhere, go for that.
Benchmark comparison click here - an overclocked 8800GT is 12th from top while a stock X1650 Pro is 8th from bottom. G