This is my first post, and I don't claim to be an expert trouble shooter, but here're a few ideas I'll throw out. Please, ANYONE, correct my mistakes if you see problems with my suggestions.
If, as you say, PSU voltage is OK and all fans spin, but there's no beep codes, you MAY have a dead proc. or a dead mobo. However, I wouldn't jump to that conclusion right away, because it may be that your case speaker wire is disconnected from the mobo front panel, in which case you wouldn't be able to hear beep codes. Or the speaker wire may be rerouted through the system sound chip (either on-board or sound card.) But if your system beeped before and doesn't now, the speaker connection is probably OK.
Be aware that beep error codes normally occur during BIOS hardware check BEFORE the video card is detected. That's why they have audio beep codes instead of POST screen error codes early in the boot-up.
(1) Check your case speaker wire and make sure it's connected to the correct pins on the mobo front panel. That way you'll know if it's POSSIBLE to hear ANY beep codes. This connection will probably be OK.
(2) Look in your mobo manual, (or find it at on-line manufacturer's website) and see if there's a jumper that needs to be reset to disable the the on-board video, or alternately enable the PCI video card. Does your mobo have AGP capability? Sometimes a newer BIOS will be able to detect an AGP video card even if there's no jumper is for disabling on board video. It may not work, but it's worth a try if you don't have a jumper to reset, because with no reset jumper the BIOS will often try to detect another video card before on board video. It would be helpful to know what general type of BIOS you have.
It would be very helpful for those of us trying to offer suggestions to know the make and model of the mobo, the chipset, and whether this is a homebuilt or OEM unit. More specific information from you could help. As a last resort try substituting another PSU or CPU, and of course make sure you get the heatsink securely attached.