Conclusion
If you want to move to Socket 775 and PCI-Express for your Pentium 4 platform, you need not replace DDR memory immediately. If 2GB of memory is enough for your purposes, you can choose from a variety of motherboard products which support DDR and DDR2 memory. However, if you want to continue to use a high-performance AGP 8x video card, you must be prepared to sacrifice AGP performance or determine the real-world impact of limiting PCI-Express bandwidth. The choices are yours, so choose wisely.
For Further Research
Chipset Vendor Websites
Intel 9xx-Series Chipset family http://developer.intel.com/design/chipsets/express_flyer.htm
VIA Technology
VIA Technology PT880 Pro http://www.viatech.com/en/products/chipsets/p4-series/pt880pro/
VIA Technology DualGFX Express http://www.viatech.com/en/products/chipsets/dual-gfx/
Motherboard Reviews
Tom's Hardware reviews the Elitegroup 915P-A http://www20.tomshardware.com/motherboard/20041027/index.html
PC Stats reviews the Albatron Mars PX915P-AGPe http://www.pcstats.com/articleview.cfm?articleid=1691&page=1
Anandtech reviews a group of Intel 915P-based motherboards, many of which feature DDR and DDR2 compatibility, at http://www.anandtech.com/mb/showdoc.aspx?i=2293
Bjorn 3D reviews the DFI LANParty UT 915P-T12 at http://www.bjorn3d.com/read.php?cID=738&74182