Windows 98SE is typically the oldest OS supported with any new game, and more importantly, drivers. It's important to note that Microsoft will be discontinuing support for Windows 98 later this year, so future Direct X revisions (and thus gaming support) will not be around much longer for this OS.
Windows 98 (first edition) is no longer supported.
The short answer on the ram is "no", but that's not the whole truth. Your memory is going to be PC100, or maybe PC133, SDram. There was a PC133 supporting i845 chipset for the P4, but its performance was so bad that there was literally a class action lawsuit against HP, Compaq, and Gateway because of it. (The resolution was credit for those customers who had one of these.) PC133 SDram so badly bottlenecks a P4 that it was a very poor choice to use this type of memory. This chipset is also now obsolete, and does not support any P4s currently in production.
Personally, I'd recommend a Springdale (i865) chipset based board, like the Asus P4P800-E. These are very fast boards which support the newest "C" type P4s (800 MHz FSB with Hyperthreading support, available at 2.4, 2.6, 2.8, 3.0, and 3.2 GHz). I'd recommend a 2.4C - 2.8C, as the higher end CPUs are quite expensive. This board, as with any newer Intel P4 supporting board, is going to use DDR SDRAM. You should be using DDR400 if you can afford it, but DDR333 is acceptable. Get two matching sticks of memory, either 2x 256 MB for 512 MB total, or 2x 512MB for 1 GB total. 512 MB is typically more than sufficient for most computers today. (It's worth noting that Windows 98 will report an out of memory error if you exceed 768 MB.)