there is no benefit from running a 64bit CPU on a 32bit environment, like WinXP. the reasons for why athlon64/fx’s are outperforming the AthlonXP for example, are pretty diversified.
the transistor count for the 64 and the FX has nearly doubled over the XP - responsible for that is the increase of the massive Level1 + Level2 Cache, which are of course, on DIE.
the L1 cache weights 128k and the L2 1024k - the XP has a 512k L2 cache.
furthermore, the memory controller is no longer located on the motherboard’s northbridge (which was completely moved on-die), but on the CPU DIE itself (64+FX), which dramatically reduces latancies during memory transfers.
apart from that, the Athlon FX supports dual-channel (registered) DDR, whereas the Athlon64 still runs them in single channel mode.
but all the architectual improvements made to the core and subsystem, improve the performance also for 32bit dramatically, without sigificantly raising the nominal operating frequency.
both, the athlon64 and the FX do also support intel multimedia extensions, SSE & SSE2.
nowadays P4’s get constantly outperformed by AMDs 64/FX CPUs, only when hyperthreading is supported, the P4’s are able to perform equally good or better. so if you’re not into video editing/encoding or plan to overclock the hell out of your P4, i’d definately go for an athlon64, since it really offers the better performance for nowadays most common applications like games and also, and i guess that’s the point here, renoise.
price/performance wise, i’d go for an Athlon64 3200+, the one running at 2Ghz, featuring 1024kb L2 cache. costs around 200-210EUR here in germany. you won’t be disappointed by its performance.
(the athlonFX is pretty out of your price range, i guess. the FX-53, the current flagship model of AMD, costs around 700EUR++)
memory-wise, you should go for at least one stick of 512MB PC400, but if your wallet allows for for a further one, it wouldn’t hurt of course.
concerning the graphics card, i wouldn’t go for a 9600XT, especially as you plan to play games like FarCry, which is up to now, the most hardware demanding piece of gaming software out. the 9600 (no matter if XT or not) only features a 128bit memory interface and 4 rendering pipelines. a 9700(pro) and 9800(pro/xt) all feature 256bit/8pipes, which really pay off in graphically intensive situation.
so my recommendation would be to get a rather low-cost radeon9800pro now, as prices are really affordable for these cards meanwhile - for around 180EUR you should be able to get one of those, or a pre-used 9700pro, for around 100EUR is a good one as well.
hope this helps a bit…