Will the revodrive X2 240GB work on my Gigabyte EX58-UD5 motherboard which has a PCI-E 2.0x16 slot ?
The specs for the SSD say it needs a PCIE 4x slot, but I'm wondering if this is a different slot or a different speed (ie will it be backwards compatible with PCIE 2x, or do I need to buy a new motherboard ?
The answer to your question is yes. The OCZ Revodrive X2 solid state drive will work in your motherboard's PCI-e 2.0 (or 2.1) x16 slot. The motherboard slots are backward compatible.
The x16, x8, x4, and x2 refer to how many lanes are available in a motherboard's PCI-e slot to transmit data or how many lanes a device such as your ssd requires to function properly. In your case the motherboard slot has 16 lanes available and the ssd only requires 4 of the 16 lanes. No problem.
The 2.0 and 2.1 as in "PCI-e 2.0 x16" refer to revisions in the PCI Express international standard. The revisions should not affect ssd performance.
If memory serves, there was more than one version of the Gigabyte EX58-UD5 motherboard. Please check your motherboard manual to determine which type of PCI-e slots your motherboard has. I seem to recall one version also had a PCI-e x8 slot and a PCI-e x4 slot in addition to two PCI-e x16 slots. If you have that model, then you could use the x4 slot instead.
There is one more thing I should mention. There are some motherboards that have two PCI-e x16 slots. As long as you only use the primary slot it has all 16 lanes available for the primary slot. That primary slot is usually for a video card. However, if you add a second device such as a video card for Crossfire or SLI or a video card and a ssd, then both x16 slots are reduced to only 8 lanes each. It's like both slots have to share 16 lanes. The video cards will suffer a slight performance hit of about 2% to 5%. Please check your motherboard manual or Gigabyte's web site for more information about your specific motherboard. Hopefully you'll get lucky and have all 16 lanes available in both slots.