You can probably fit a PCI wireless card in there.
Open side panel and see if yours has an AGP graphics slot (Dell did cut it out from most PC's)
If it has then you can upgrade to geforce 5XXX series or ATI 9XXX or X1300/1600.
I think there used to be PCI Radeon 9600 but that might be a bit too slow for games.
It uses old SKT478 CPU so most P4 CPU's should work.
Should also max out at 2GB of DDR RAM.
Hope This helps