icepick...when you buy a case, usually they'll say if its ATX or another standard, such as the older AT standard.
To know for sure, you'll have to look inside the case at the Power Supply connector. If its 2 long white strips, its an AT power supply, if its 1 single white long strip, its an ATX power supply.
As for chipsets, I don't think you understand them well. Chipsets are non-upgradeable. A motherboard only has 1 chipset. For the intel camp, there's the venerable intel BX chipset, the newer intel 810, 820, the new intel i815 chipset, and the VIA Apollo Pro. Performance wise, the BX still rules, but you'll have to overclock it and run several components out of spec, since there's no official support for newer features such as 133MHz FSB and 1/2 AGP divider. There's also no official support for ATA66/ATA100, but most new BX motherboards do include that feature.
In the AMD Athlon camp, there's the old AMD750, the newer VIA KX133 and the latest KT133. Depending on which CPU you're getting, you should choose between the KX133 and KT133, since the AMD750 is getting old and lacks several features. Note that the KX133 does NOT support the newer Duron and Thunderbird CPUs.
As for processor speed, you should really get more than 450MHz if you play games, even at higher resolutions. Personally, I'd recommend at least a 600MHz CPU, depending on your budget.