Go here and install the hard spl:
http://forum.ppcgeeks.com/showthread.php?t=93252
It has a step by step instrucion for it. Basically, download it, make sure you are connected to acticesync (or windows mobile device manager in vista and newer) and then run the exe. it does the rest.
After you have the hard spl intalled, you can install whatever ROM you want. You do that the same way- download the ROM, make sure you are connected to active synce (or windows mobile device manager) and run the EXE for the ROM you downloaded.
The ROM I am using is Indagroove's Titanium. It works really well with Mobile Shell. Way faster than the stock ROM, finger friendly WM 6.5, and it's a Sprint ROM (and I have Sprint so it's perfect).