The GT shouldn't block the adjacent PCI slot...it's designed to be one slot only...but I've heard that it can interfere with some cards...not that you would want to fill that adjacent pci slot anyways...blocks airflow to your vid card...
It really depends which brand you're getting. My PNY only takes up the AGP slot but its pretty much flush against the next PCI slot. I could probably get a PCI card in there if I had too but it would really restrict airflow.
The GT with the reference HSF is going to be just like any of the ATi cards...the only ones that no-shit take up 2 slots are the Ultra and some GTs with custom HSFs...