i own the SC and it's an excellent card...i previously owned an MX300...
i have a few complaints:
1) WDM drivers don't allow software independent mixer control (i.e., if u adjust the mixer in winamp in win2k, it adjusts the WAV mixer for everything...it doesn't do this with the VxD drivers for win9x)
2) the tbcspud.sys driver file crashes my comp when doing chkdsk for NTFS partitions in win2k during boot...renaming the file (i.e. deactivating it) remedies it...but it's quite a pain in the ass...i don't know if it's specific to my system or not.
3) A3D is relatively quiet..requires jacking up the volume a little...it's no so quiet u can't hear it, but it's quieter than my MX300 A3D and the SC using EAX or nothing...
i'd assume most of these issues are present w/ the GTXP (except maybe (2)), though i'm not certain....
all in all, a good card....love it...
i like the way u can switch channels conveniently (i.e. to use front only, back only, four only, versajack, etc.)...the analog is crisp and clear and the win2k drivers work flawlessly (w/ the listed exceptions)