|"How different are the GMA X4500 vs 8400gs?"|
The 8400GS should be slightly better but it depends on which one you have. There were 3 different versions - the original was based on the G86 GPU, the 2nd was based on the G98, & the 3rd was based on the GT218. There were both PCI & PCI-e cards, plus some used DDR2 & others used DDR3.
If your card has the weakest GPU, DDR2, & it's PCI rather than PCI-e, I would expect the performance to be very similar to the X4500. If you want to play games & play them well, I suggest you dump the slow/outdated 8400GS & invest in a decent PCI-e gaming card with GDDR5 memory.