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Hi there,
May I ask what you will be needing the GPU for?
If its just basic stuff and HD video decoding - then I would suggest waiting for the HD 6450 (review here) or just get a passive 5450 now.
If you want to play any kind of games, then I strongly recommend you look for a second-hand card. Cards like the 8800GT, 9800GT, 9800GTX(+), GTS 250, ATI HD 4770, 4830, 4850 can be had for than kind of price and completely blow cards like the 8600GT and 5450 out of the water in terms of gaming performance.
It looks to be a replacement for the 5450 - so I would expect £35-45 initially.