whoever told you that you can only use pc3200 RAM is an idiot....As long as it's DDR RAM it'll work in the board...but just realize that it won't just pop in and run at PC4800 speeds...it'll default to PC3200 speeds of 200MHz, then you'd have to go in the BIOS and change it to 300MHz which would be PC4800 speeds...
Mind you you'd need to change your LTD bus from it's standard 5x multi down to 3x since your HTT will be so high....I believe your stock clock speed is 2.2GHz with that chip so you'd need to lower your CPU's multi down to like 7, that'd be 300x7 = 2.1GHz though so that'd be underclocking, or you could use 7.5 and achieve 2.25GHz which would be a 50Mhz overclock which IMO would be better than a 100MHz underclock.
So to recap:
HTT = 300MHz
Make sure to use a 1:1 ratio with your memory so it's also at 300MHz resulting in it actually running PC4800 speeds
LTD Multi = 3x
CPU Multi = 7.5(for 50MHz overclock, shouldnt be a problem with that CPU, maybe tap it's voltage up .25 but I doubt it'd really need it, it's up to you to check for stability of course)