Given that it is a Pentium 4. We'd need to know which Pentium 4 and what motherboard it has. If it is an OEM like Dell or HP. Which specific model number. So, don't just say Dell Opteron. We'd need to know something like Dell Opteron 390 MT. Also if the case is small form factor or mini tower.
Given it is a Pentium 4 it could use an AGP or PCIe slot. Some cheap models just had PCI slots.
As for upgrades. Any card would be used. You'd really be limited to mid 2000's games. Maybe some late 2000's.