CPU motherboard doesn't use any signal in 3.4GHz range.
All signals on motherboard are much slower. High speed CPUs
internaly generate higher clock. This has been common practice
since 486DX2 and that's close to 20 years.
DIY counters for gigahertz range are not much of a deal anymore
because of technology advancements.
Just take a look at
http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=232884
It's a $2 prescaler chip that runs up to 3GHz.
Simple frequency counter using LCD and microcontroller can be
made for up to some 50MHz. Now if you divide the input signal
10, 100 or 1000 times you should be able to use that frequency counter
to measure much faster signals (10x, 100x, 1000x). Don't take
my word for it, take a look at this:
http://www.qsl.net/n9zia/pre/
There are faster prescalers as well, like Zarlink that works up to 13.5GHz
but I didn't bother to look up pricing:
**broken link removed**
The whole 3GHz frequency counter project could cost as little as $50
(at least for the hardware, don't know how much you value your time).
Since most guys with some electronics knoweledge and internet access
coud do it, I wouldn't consider 500MHz counter project a candidate
for Nobel prize.