As you have a Radeon card, SA2 will "work". It will still crash after the first level, and randomly at other places in the game (which you can get to by using save games).
P4 1.8 is much to slow for full speed emulation of a dreamcast, sorry - 1.8Ghz is a bit low in any case, and the earlier P4's especially underperform. Also, note the famous thing most new people miss: DC games (GD-Roms) cannot be put into your cd/dvd drive and played.
If that hasn't turned you away, you can have an excellent time using chankast, its an amazing emulator.
Edit: missed by a min. Good work getting it to run, and better work being pleased by the results

One can never tell which type of newbie one is!