@ mactronix: I have not seen the AGP HD4650 here in the UK but it is on the Newegg site.
@strangestranger: Yes, on both points.
@ GPotvin: As mactronix says, the HD3850 and HD4650 are about the same in performance, but the HD4650 uses less power, often an importaint point in older AGP systems with-usually-older and weaker power supplies.
It is really now up to you which way to go. A new card will give a nice boost but the game will always be limited by the CPU.