I agree with the last poster, that it will likely be a bit faster that way. But if it were me, I'd probably just image the 80GB drive to the new SATA and ditch the 80GB altogether...or maybe just keep it around for backup purposes. You will be happier with your overall system performance and your swap file will be faster.
Of course you could move your swap file to the new sata, leaving the old stuff on the 80GB...but that would also lead to some contention with your games...some...
In general though, I would just move everything to the new SATA. If your 80GB is full, then it's likely slowing down...and will continue to do so until it's completely full. Your swap file is likely slower because of this, leading to slower general responsiveness.
In my experience, I have noticed the biggest general performance gains in my system, by optimizing the hard drive layout. I have a RAID 0 for my boot and it flies.
That's my 2 cents at least.