First thing is to add more memory, at least 512 MB and even more if possible. You may want to check that the system can handle more than 512 MB without slowdowns.
Second thing is to replace the disk drive with something bigger and faster - preferably flash. This should speed up Windows XP substantially, since common activities (boot/shutdown, starting applications and loading data) will be starved waiting for the disk. Adding memory reduces the load if data has been loaded into cache once, but it is probably good anyway.
If you've got an AGP slot, you might want to add a decent 3D accelerator, such as a Geforce 2, which still had good driver support for Windows 98. Otherwise, PCI variants of some cards exist as well.
Your sound card will not have great compatibility for DOS games in DOS. If your DOS games run well in Windows (either 98 or XP), this is not a concern. Since your system has no ISA slots, there is not much which can be done.
According to Wikipedia, the system supports up to 866 MHz, so you can upgrade the CPU to improve game speed. Depends on the games you target whether that is worth the effort - I personally consider memory and disk upgrades more useful immediately. Keep in mind that a Celeron is a low-speed processor; if I had the choice I would stay away from those.
