That's a very good choice*, but what's the rest of your system like - in particular the CPU?
If it's not an Intel dual-core E4500/similar or better, the HD3850 could be bottle-necked by the CPU (i.e. won't perform at its true potential). If this is the case, then you may be better off going for a X1950XT/7950GT/similar depending on your CPU. eBay search for X1950XT & 7950GT click here - I wouldn't pay more than £60-70 delivered.
Also, at least 2Gb RAM highly recommended.
Cheapest AGP HD3850 I could find is £116.50 delivered from Aria click here. G
*A comparison between the HD3850 & couple of older gen cards click here. For a more comprehensive list click here.