The speed of the PCs, the Manufacturer and model of the Network Interface Card, the quality of the cable & connectors will all affect the overall throughput.
Even if you have a 1.2 Gig Athlon, if you put a cheap NIC in it, you'll never see ultimate throughput. That's why cheap NICs are Cheap.....they suck perofrmance-wise (though they may be "great performance for the price".
Lots of folks go out & buy pretty decent CAT5,5e, or 6 cable...then crimp on UNRATED mod plugs...effectively making the cable CAT 3 (weak link rules apply..the path is only rated as high as it's lowest rated component). Some folks don't know that there are different mod crimps for solid-conductor and stranded-conductor cable...using one type on the other type cable may reduce the category rating (if you get a good connection at all).
Also note that a "Wire Speed" switch may accomplish "Wire Speed" through buffering...increasing latency and causing problems for real-time applications. Most vendors have their own definition of "Wire Speed"...read carefully.
Overall network speed is also going to be affected by the protocol in use. If the application is using TCP, the sending machine must eventually wait for an ACK packet from the receiving machine before it can continue to transmit (making it look "Slower". UDP packets are sent with no expectation (at that layer) of any acknowledgement...so they tend show as "faster" (and used for testing/benchmarks, since it presents better throughput figures).
Most switch tests are done with products like a "Smartbits." This is testing apparatus (and software) specifically designed for throughput, latency, and packet drop testing (and many other functions). Because of the way the Smartbits sends and receives the data, it will always show better throughput figures than a couple PCs blabbing at each other though a switch.
If you want to try a pretty slick visual throughput tool, find a copy of "Bricks." It was written by DEC (Digital Equipment Corp, now defunct) guys for use at trade shows to show throughput visually for demonstrations. It can be set for TCP or UDP packet types. basically it has a pile of "Bricks"...each "brick" contains X number of bits (1Meg, 10Meg, etc)...as each increment of bits get transmitted, a brick is pulled off of the transmit pile and added to the receive pile. Multiple transmitters can send to a single receiver to show reletive aggregation ratios (stream / stream). The program is Public Domain and shareable.
FWIW
Scott