That's because there was no GX250. The GX2xx series was the GX200, the GX240, the GX260, the GX270, and the GX280.
The GX240s were really slow for the very reason of using SDRAM (oh, and Williamettes). The Pentium 4 was finally able to sell well to the mass market once OEMs started using SDRAM (RDRAM was expensive) with the 845 chipset, but it was barely faster than top end Pentium IIIs of the time. The Pentium 4 needed alot of bandwidth. It wasn't until DDR came around and Intel dropped RDRAM that the Pentium 4 got a boost.
The GX260s will probably be the better ones (Northwoods, DDR, AGP, etc.), unless the GX270s are fine from the issues I mentioned, then they'll be a bit better, but the differences between it and the GX260 weren't as big, if my memory serves me right. Those two models could be "solid" machines.
The GX280 made the switch to LGA775, PCI Express, and I think DDR2 as well, but the base of the platform was otherwise similar to the GX270 too (I think it had the 915 chipset, so no dual cores at all, just an otherwise similar Pentium 4 platform).