Back in 2008, I upgraded from a 1950Pro AGP to a HD3850 AGP. It made a significant difference @1280x960 gaming, in particular with Red Alert 3 and Assassin's Creed. Correct me if I'm wrong but the 3850 is supposedly the fastest AGP card available. The 4670 is not faster.
Given the system age however, I'd personally be hard-pressed to spend more than $30 on an upgrade card. As was suggested initially, it is far more cost-effective to buy a CPU+Mobo combo.
Cheers!
AC is from 2007, no surprise a GPU from 2007/2008 beat the 2005/2006 GPU by a decent margin, but realistically, if you are building a retro gaming AGP PC you should be targeting at older games, and in that case something as new as the 4670 might give you more headache with compatibility issues than benefit compared to the older card... if it's for playing 2007+ games, yes, just get a PCIE system.
the 3850 is the fastest or almost, it had 256 bit GDDR3 while the 4670 had 128bit memory, but the 4670 had improved efficiency, higher GPU clock and 32 TMUs vs 16... so it depends on the game, a 4670 with 1800-2000Mhz memory can beat the 3850 in many games
did 98 even have internet?
I've used Internet on Windows 95 (with IE 3.0, 4.0 or something), and it existed before that... you can still run some relatively new web browsers on win98 (I think I have Opera 9.64 on my win 98 PC, and also "K-Meleon" from 2014 or so, I think Firefox when I tested was to slow, but worked)