When Compaq / HP brought out the V6000, the HDD sizes they provided were 80-200GB, but this was down far more to available capacities and costs than for technical limitations - this was 2007, after all!
See:
www.hp.com/ctg/Manual/c01035886.pdf
If your system drive controller supports SATA drives, then it supports 48-bit LBA and can support HDDs much larger (PB size!) than those originally specified by Compaq. Of course, if it is ATA 5 only, then you might be out of luck - but that's highly unlikely in 2007.
The other thing is the BIOS limitations - before 2002, 137GB was the limit, after that most BIOSs supported up to 2TB, so your BIOS is likely to support 2TB disks, as will Vista.
So assuming that your system has a SATA HDD, you can install a disk up to 2TB.
SD