You only need a PCI slot on the new board, if there are any functions the new board lacks, but that you have PCI card(s) to accomplish. Even then, there are a few reasons to try to avoid using PCI slots such as finding drivers if not built into the OS, the lower bandwidth available, latency, and general age of the PCI cards (nothing lasts forever). Plus a board with only PCIe slots, has more forward looking compatibility and options on where to put each PCIe card, for example leaving some space around the video card for better airflow.