Cat 5 is good for 10/100 ethernet.
Cat 6 is good for gigabit ethernet.
Of course your hardware needs to all support the minimum standard you are trying to achieve.
Some ethernet cables I have found to be real cheap, as those provided with some internet providers manage to stick to the ethernet port of my old Powermac 7200/75. Ethernet cables with all metal clips may have a greater chance of doing that. Note, legacy equipment on this board refers to pre-PowerMac. Your MacPro is post powermac. You are better off posting in the Mac OS X technologies or the 10.6 forum for future questions.
Quality of ethernet cables within a category does vary. Considering a cable for $5 when most sources are in the range of $20 should be a warning. You don't need to buy the most expensive you can find, but stay away from those that are significantly below the majority of prices you see. Connections may not be tight, number of twists per meter may be off, and so on.
Do you get what you pay for here?
You likely won't get what you pay for with the most expensive ones.
I've had good luck with every cable I got from this place...
Seem higher quality than the most expensive ones.
As far as network cables are concerned, they will work until they don't; that is, there really isn't any performance gain you will encounter from using one type of cable over another, so long as it works with your network setup. Generally, the speed of the network device you're connected to (10/100/1000 Mbps) will determine what type of cable you need; Cat5e and Cat6 are certified for 1 Gbps LAN, while Cat5 is limited to 100Mbps, Cat3 to 10Mbps. However, some manufacturers make cables that exceed the published standards for crosstalk, as I've seen Cat3 cables that work fine on 100Mbps networks and Cat5 cables on 1Gbps. Your network card will automatically negotiate the speed it needs for the network it's connected to; if you had a bad cable or one that's not rated high enough, it shouldn't work at all.
I've had bad luck on my own with ethernet cables, just once, with customers several times. My ethernet cable I got from Comcast in the early days of Cable internet was so thick it could not be pulled out of the Powermac 7200/75's port without a pliers (sp?). Of course this nearly broke the port once I managed to get it out. Make sure if you get one with transparent ends, that all the colored wires that are supposed to be there are. If you have any really old equipment, knowing the difference between a crossover and non-crossover ethernet cable is important.
Edit: Lastly, with many older cables, the ends can come loose, or the tab that snaps it into place can break off. So be careful.