As you can read in the P4 thread I have in my sig, I bought a Radeon HD4650 AGP the other day. The drivers are a bit messy, but it's doable.
It's true that both an HD3850, 4650 and 4670 are going to all be bottlenecked by the single core CPU, but there are some advantages to the HD4xxx series:
- more Vram - the HD3850 AGP only comes with 512MB. Why is this important ? You can use more Anti-aliasing on the HD4xxx series cards - and this series works better with AA too;
- less power consumption;
- better video features. Supposedly it can even do hardware acceleration of HD Flash - you're going to need it to watch HD Flash videos on Youtube. The Pentium 4, even overclocked to 3.3 is not going to be smooth all the time - I now because I have a 3 Ghz P4, and now a 3.4 Ghz one. It can play smoothly, but not while it is still loading the video - the CPU doesn't have the horsepower manage both at the same time (after it has downloaded the whole video it gets smooth).
The AGP drivers are a bit of a nightmare. The ones included on the driver CD do work, but in my case, it's Catalyst 9.6, which does not have the HD flash acceleration features. The one on Gigabyte's website also works, but it is from September 2009, which is barely newer than the one that came on the CD.
AGP hotfix drivers 11.3 give constant reboot loops to many people; you can lookup one of the last thread I wrote on the matter in the P4 thread.
You can however get the Catalyst AGP Hotfix 10.10 from Softpedia. Those work, give a performance boost in games; but at least in my case the Catalyst Control Center of this version doesn't work. You have to install the one that came with the CD, and then only install the 10.10 driver, without the CCC. So far it's been working for me. I still haven't tested if the HD flash hardware acceleration is working, but I'll do that tomorrow. If you want to be on par, also with benchmarks, you can check the P4 thread.
As to the 6800GT -> HD4670 upgrade, yes it is a big upgrade! I can actually play Crysis with a Radeon x1950 Pro with good image quality, and now with the HD4650 and the 1GB framebuffer I can set the shaders to High and use 2x AA, which improves image quality even further. Games such as Left 4 Dead also play fine (you have to set the paged pool memory to low to avoid hiccups).
In general you can get new games to work as long as you set the CPU intensive stuff to Low.
If you are using Windows XP you can consider buying a used Sound Blaster Audigy 2, 2 ZS, 4 or 4 Pro, as those have hardware acceleration for sound processing and offload that from the CPU. You can gain an additional 2 - 4 fps. Given it's a single core CPU, every help counts. If you're using Windows Vista or 7, it doesn't matter as they don't support hardware acceleration for sound processing (the CPU's nowadays are more than powerful enough to handle sound and the game calculations)
Edit: if you want, you can also upgrade the ram to 3GB, I'm assuming you now have 1+1 in Dual Channel. You can get an additional 2x512MB to go for 3GB. In games it will make a difference. It is not worth it going to 4GB, as Windows XP (or any other 32-bit OS, for that matter) will not be able to use it, as the 1GB of Vram of the graphics card has to be mapped to the main memory address space (which is 4GB for 32-bit systems), effectively giving you 3GB of usable memory. So, 3GB is the sensible and cheaper option.
