why do you want to purchase an SCSI HD for gaming? it will not improve game performance, it will merely make levels load faster. i personally think its not a wise choice, but if you have the money and really want levels to load faster then by all means go for it.
just an added note, if you do have money to burn on a high performance HD, i recommend the WD raptors. they use a SATA interface so an SCSI controller card is not needed. (unless your using, or plan on using, a mobo with native SCSI support.)
15k SCSI HD 300gb - $655
10k SATA2 HD 150gb -$225
7.2k SATA2 HD 320gb -$70
Cost per GB:
15k SCSI HD - $2.18/gb
10k SATA2 HD - $1.50/gb
7.2k SATA2 HD - $0.21/gb