As has been said, buying the 3850 AGP is false economy. You're pumping money into dead technology. Personally I believe Crysis to be quite CPU-limited - it's pretty terrible at using multi-core, which would explain why it only uses 75% of your P4 if it has HT. However, if you bought a cheap C2D and clocked it to 3Ghz, each core would be almost twice as fast as your P4 - that's gonna make a big difference in gaming but will blow your P4 into the dust in terms of video encoding.
If you can really only justify £150 then you could probably do worse than the 3850, but to me it just seems like wasting money. The 3850 AGP is a bit of a niche part - that's why they can get away with charging £150 for it, and why it will have very little resale value when you do come to upgrading to PCI-E.
nVidia would say that
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It depends what type of games you play, and which resolutions you like to play at - and, of course, which CPU and GPU you have. With a 3850 and P4 I'd say that most games would be heavily CPU-limited at the resolutions you're likely to be playing at.