Hi
Before considering a new case and psu
Many Dell motherboards are non standard for mounting on pillars , often fitted onto non standard base plates with non standard mountings
Cables for usb, power switch & led , front microphone & headphone plugs are non standard
Cooling fan for cpu may not be mounted on cpu heat sink and cpu heat sink not mounted in standard manner for socket 775 core 2 duo or quad
I believe the cpu for tower models can be replaced by standard design psu's
But check rating of Dell cpu and if a 6 pin PCI-e plug already exists
The Intel integrated graphics of that generation are not going to be suitable for first person shooter type games at any resolution
They were made for business use , Internet browsing , office etc
Unless you got a model with a graphics card
Dell have good user guides & service guides on their website which will give you some idea what is practical in upgrades
Regards
Mike Barnes