spannersatcx is correct - there may be hardware incompatibilities to worry about.
When it comes to RAM - older pentium systems typically use 72-pin SIMMs, while newer systems use 168-pin DIMMs.
As for video cards - if you're upgrading an older system the most likely problem would be that the old video card fits in a PCI slot so you wouldn't be able to take advantage of the AGP slot on the new board.
If your old sound card or modem is ISA form then you could be in trouble if the new board doesn't have an ISA slot.