I guess that you want to keep your current video card... that why you want AGP. Nforce3 was the nvidia offer for AGP system. That what you should look for. Try to get the one that has all the feature you need at a reasonnable cost.
Nforce4 chipset is for PCIe graphics card. You will see some hybrid chipset supporting both, but this is only under certain circumpstance.. This is not a real AGP port you'll see there. But rather an AGP slot bridged to the PCI bus by some kind how trick.. What that mean, it will perform very bad and might not even work with your video card.. Stay away from such thing.
If you're on a tight budget, you could consider socket 754. The CPU is clocked 200MHz higher that the 939 counterparts, meaning that the 939 3200+ you plan to get run at 2.0GHz. The socket 754 3000+ run at 2.0GHz and perform about the same... maybe a bit less, but only benchmark could tell..
Socket 754 mobo with nforce3 are easy to get with a wider choice. They dont require dual channel memory, so only one stick of memory will goes well ..
Going the 754 road, keeping your old stuff makes more sense to me, as it will be cheaper and when it wont suffice, then you'll be able to sell it and start from scratch with up to date componenet instead of always having a slow one in your fast system.
remember, the overall speed is not determined by the mobo/cpu, but by the slowest part of your system... For example, an old HDD wont run faster on a new motherboard... and having good hdd performance is what makes your system feel fast.
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