When I started reading the review I thought the 9800Pro was beating the 6600GT, until I realised it was just the vanilla 6600 card. Scrolling down, the 9800 Pro gets a kicking in any benchmark that isn't CPU limited at the 1024x768 test resolution (i.e. 3DMark2001 and HL2).
If you are thinking of upgrading then I think the gap between the 9600XT and a 9800Pro is not enough, although you would notice it. Considering you can get an AGP 6600GT for £133 (OcUK special this week), only about £10 more than a 9800 Pro, the nVidia option is a no-brainer. Check out Anand's article on CPU scaling with HL2, came out today (written by the man as well, it makes a difference). After you get over the disappointment that those who can afford the top-end Athlon 64s really are getting a decent frame boost, you get to the interesting real-world options where you see how performance scales with more modest CPUs and GPUs. The 9800 Pro hits a plateau pretty quickly as you raise the CPU speed, but the 6600GT just carries on giving the love.
I was thinking a heavily CPU dependent game like HL2 would benefit from me upgrading my Athlon 2400+ to a 3200+ say, but with my 9800 Pro I now see it would be a waste for gaming. Basically, next stop for me is going to be a complete system overhaul. As for your options, I might consider moving to a socket 939 platform and buy a PCIe 6600GT. Take advantage of the extra headroom on the card while also not buying an AGP card that will be junked when you do eventually get a new CPU/MB.
Just to reiterate, there really is no comparison between the 6600GT and the 9800 Pro, much as it pains me to admit it. I bought my Radeon exactly a year ago for £135, it's encouraging to see that it's taken till now to find a competitor at that price, AND that I'm still more than happy with its performance.
If you've got your money for nothing, who cares if the chicks are free!