I agree that if it was incorrectly jumpered to start with, then theres no reason why that should suddenly become a problem now particulary.
Its not clear from your post whether the tech who told you it was wrongly jumpered from the start, is the same tech that built the PC originally?!
Also, assuming I do have you mobo make correct (ie biostar p4x266) then all the USB ports are version 2.0. This means that he was wrong about the front ports being the wrong version. Using the rear ports in preference to the front is recommended anyway, as the front ports tend to suffer more problems.
The noise you heard when you connected the camera to the usb port, was possibly the fuse blowing (the rear ports are individually fused).
I don't think there is a problem or issue with the PSU though. The voltages supplied by the psu are fixed, not anything the tech could have changed.
The reason I'm asking you to check the connections etc that I've specified previously, is to ensure nothing inside (eg USB & firewire headers) is incorrectly connected. Because both USB/Firewire are powered connections, hooking them up wrong can cause obvious problems upto & including physical damage to the mobo.
Re. your last query about new PC etc...
Firstly, your system is still currently working. The worst case scenario would be a new motherboard but you should be able to keep everything else, so this won't be as expensive as a new system by a long way.
If you do want to replace the entire system (either now or in the future), theres a couple of things to be aware of, re. custom built/private/company etc
1.Personally, I would recommend a custom build from a private tech. (I'm probably bias though as thats one of the things I do myself). BUT they must be very good at what they do. Too many people throw PCs together on the basis that they 'know what they're doing'. Assembling components is easy but selecting ones which are 100% compatible together, getting them all to work together smoothly & fixing it when they don't, is a whole different skill that takes years to learn.
But if the tech is good, then you will get a far more personal & flexible service than you would from a company.
2. If you buy from a company, even if you have some say over the components, do not buy anything unless they are prepared to state exactly which components are being used.
EG the motherboard in particular. A lot of companies tend to use cheap boards that have integrated graphics, audio, lan, modem etc. This means you are stuck with whatever is built into the board. Although any integrated components may be disabled & off-boards ones added to replace them, you normally only get a couple of PCI slots with these boards, which limits your upgrade possibilities.
Also, these boards may not be upgradeable in terms of CPU etc. ie if the board is already running the fastest CPU it can handle.