Real Tracks & Real Drums are enhancements to the styles. The Mega Pak contains all 77 styles but will be limited to MIDI for those instruments that you don't have a Real Track or Real Drum file.
I'll assume this is a first time purchase. If you had checked here either asking or doing a search you'd find that the reccommendation from all of us old timers is to buy nothing less then the Ultra Plus Pak. Save up the extra $200 and go all the way.
As I said if it's been less then 30 days they'll upgrade you or take it back.

John
ESI Gigaport HD+
Lenovo Turion II /4 Gig Ram/ Win7x64 be
15.6" Monitor
"The only Band is a Real Band"
www.wintertexaninfo.com/BANDS/JohnnyD.php