Tthe graphic card as it could easily fail. The 2007 Mac Pro 2,1 came with 7300GT OEM and more than 7 yrs ago when that card was even "new."
You can only do Recovery if you have Lion. So that helps tell us something.
Can't stress enough, invest in some bootable backup clones.
keyboards and USB cables and devices can cause mischief if nothing else. The fact you could ZAP PRAM/NVRAM (not delete, it can't be, only cleared).
There is no install DVD for Lion, but it is useful to have made your own flash installer wtih the Lion .ESD package.
If you replaced the 7300GT in the past with say Apple ATI 5770, then no DVD exists as you would need 10.6.5+
To open the optical drive door, hold down the mouse during startup. Hold Option key also to stop the boot.
Between TimeMachine and Carbon Copy Cloner and having redundant and multiple backup sets, except for a total meltdown, you have your system and data preserved (and a UPS, we just had a 30 second outage this AM for no good known reasons... all over town).