Hey Melissa,
I arrived in Nairobi in February and am living here at the moment. I'm from the UK.
Nairobi is a great place to live, lots to do, lots of fun and warm friendly people - I think you'll enjoy your time here. Accomodation can be pretty expensive - as I found to my surprise when I got here.
Karen may be cheaper as it's out of town but you would typically pay 40,000KSH per month for a 1 bed, maybe 55,000 to 60,000KSH per month for a 2 bed place. It does vary and you can shop around and haggle too.
If you need any help finding a place or any other info, just drop me a line - be pleased to help.
N.