I'd say use 5.50 GEN-B. You can find that here http://www.codinghs.com/forums/showthread.php?t=10030
I think you might need to be running 3.9 M33 or greater to be able to upgrade, so you could upgrade to the latest version of M33, 5.00 M33-6 (http://www.codinghs.com/forums/showthread.php?t=5061) and if you don't like it, or you want to be able to play games that use the PSN for online, then upgrade to 5.50 GEN-B.

I'm assuming you already have and know how to use nitepr, so here are some codes.
for NFS Most Wanted:
http://www.codinghs.com/forums/showthread.php?t=2637
http://www.codinghs.com/forums/showthread.php?t=1043
and Midnight Club LA Remix
http://www.codinghs.com/forums/showthread.php?t=3633
http://www.codinghs.com/forums/showthread.php?t=3853