There really isn't anything worth upgrading to with that board. There is a simple solution to upgrade too though. Your particular case should be non proprietary for front panel connectors on the motherboard. I say this because I also once had a P4/ s478, Emachine and this was the case. It can be gutted and replaced with more up to date components. You can actually put in a new motherboard, cpu, and ram, maybe a PSU and leave everything else alone for now. I ran a Phenom II 560, 4gb ddr3, Biostar 760g board, an OCZ 500w, and an HD 3870 in mine for awhile.
So depends on your budget here. Even a used Core 2 M-ATX board and CPU would be far superior to anything from S478 and a good deal cheaper than new, since you should be able to reuse your ram if it is ddr2. You could probably easily do E8400, heatsink, and compatible M-ATX motherboard off ebay for less than $100