DDR3 Ram

Whilst looking at it now i see the original is 10666 and the new one 10660.
Would this cause an issue? I can't seem to find 1gb ddr3 pc3-10666

Well first off why would you buy tri-channel ram for a dual channel mobo?
Waste of money!
Two matched sticks of tri-channel will run in dual mode in the proper matched dimms slots.
The only problem you have now is an orphaned tri-channel module so you've been running 3GB in single channel mode.
Adding a 4th dual channel ram stick with different voltages is just going to further screw things up.
My advice to you.
Sell the tri-channel ram and buy a 4 or 8GB dual channel kit.
Ram is dirt cheap and you'll get full memory performance out of your mobo.

It was a newbie mistake.
The original build was going to run windows xp which wouldn't support more then 3gb(?) ram.
Whilst upgrading (and after i purchased everything) i got given a copy of windows7 professional 64bit which can support more.

My other half wants to upgrade her PC soon.
Should i make sure she gets a triple channel mobo and use it in there and just buy new ram for her?

Are all 3x 1gb running in single channel mode?
Will that be causing significant performance loss to my PC?

Well if you ran only two sticks you would have 2GB running in dual channel mode.
The 3rd stick does however give you more ram 3GB in single channel.
Personally i have no experience pertaining to performance differences.
I'd try two sticks for sure though.
To fully utilize the tri-channel ram you need to buy a 1366 X58 mobo for your wife.
2 years ago most people ran 4GB of ram on dual channel boards but 8GB is the trend now.
3x2GB was the most popular Tri-channel kit but with ram so cheap lots of folks are going to 12GB.