The 6100/6150 chipset doesn't support AGP, so you can't put the 4200 into such a mainboard anyway. The 61xx isn't going to give you very playable performance even at low settings with the newest games, but can at least play the previous generation of games at low to medium settings. It also gives you support for newer technologies for video playback (you can read reviews/previews for details on exactly what you might like about it or not).
The 4200 would almost certainly provide better performance in any games that you compare where the features are supported the same between the two, however the 6100 enables newer features that the 4200 just doesn't support. It's hard to find real comparisons between things like that in reviews though; there aren't many performance reviews of the 61xx in the first place, most of those only compare to other integrated chipsets, and no reviews of any video cards ever compare to such elderly video cards as the 4200 anymore (which I wish they would sometimes, specifically for people like this who wonder whether they really need to upgrade).
You could consider a board such as the ASRock 939Dual-SATA2. It's a cheap board, but it has an AGP slot as well as PCI-Express for later upgrades, and also can be upgraded to support AM2 CPUs if a future processor upgrade is desired.
If you must have something with new video now, then the 61xx is probably the best bet for performance. The upcoming Intel chipsets with the GMA X3000 also appear to be a nice option, if you can wait for it.
Also consider that cards such as the Geforce 6600 are not terribly expensive, and would provide considerably better performance than the 4200 and support the newest games.
Discussion Zen 5 Speculation (EPYC Turin and Strix Point/Granite Ridge - Ryzen 9000)
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