Long story short, I'm trying to revive an old P4 AGP machine into some sort of gaming rig.

I'm looking at the Radeon HD 4650 and 4670.

Now, given that I've seen benchmarks with a CPU bottleneck (for a P4) with the 4650, am I right in thinking it's not worth shelling the extra for the 4670, in spite of the faster clock speed and faster DDR3 memory?

Depends on what task the bottleneck appeared. The p4 may hold the GPU back in some games and not others. I wouldn't worry about the bottleneck and get the better card if it isn't too much more expensive.

Well it's twice the price...

I game happily on high settings at 1440*900 on Fallout 3:NV and COD:MW2 etc using a Radeon HD4650 with DDR3, something like this but mine's a PCI-E one from Sapphire and with 512MB DDR3 RAM (processor is an E5300 at stock, 2GB system RAM).

Not sure how much difference it makes 512MB vs 1GB, what I bought was just a slower clocked 4670. I think going for the DDR3 does make a significant difference, IIRC from the reviews I read at the time.

Not sure you'd be able to tell the difference buying the higher clock speed of a HD4670 (I managed to overclock mine up to the same speed with the Overdrive utility but it made no difference to anything but the power consumption and fan noise).

YMMV - don't shoot!

I think the combination of the old agp motherboard, the ram and an old single core would bottleneck alot of recent graphics cards TBH. You can only take old kit so far before it becomes a false economy.