The 4800+
should be beaten by the Intel Core 2 DUO E6700 "LGA775 Conroe" 2.67GHz (1066FSB) as that is Intel's second one down. The 4800+ has a speed of 2.4Ghz and the Conroe has a speed of 2.67Ghz which now due to Intel's new architecture seems to be better than the 2.4 from AMD (it used to be the other way round).
All this is due to Intel going back to the drawingboard for the Conroe and completely re-doing their systems so that they run effectivly without the high temps and instability.
In six months time though AMD will be releasing their 65nm chips, currently AMD only has 90nm chips and Intel's Conroe is a 65nm chip so Intel is ahead...but I recon that AMD will come out tops once again in six months time.
Forgot to mention, if you want to spend £400 on a chip, then get the E6700 as it's £411.