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Check your motherboard manual for placement of ram for dual or single channel.

I wouldn’t worry too much about not having dual channel working on your motherboard as it would only make a difference of between 1% and 3% hardly noticeable except on benchmarks, certainly not worth spending any money on.

No, that is not a good setup.
With an odd number of sticks, you will not run in dual mode. If it is a core 2 cpu, it won't make much difference.
The best is to get a 4gb kit in a 2x2gb configuration. A 32 bit OS will see about 3.4gb of it.
Also be wary of mismatched sticks. Some motherboards will be very sensitive to that. Assuming that it works, you will be running at the specs of the slowest stick.

Try what you have, and see how it goes.

If you put in the 4 gig of memory you have, your motherboard will not run in dual channel mode and you will lose 1 to 3% in performance compared with if you only put in the (2 x1) gig of memory you have. On the other hand by putting in the 4 gig you will gain about another 1 and a quarter gig of memory for your programs to use with maybe a slight improvement in the operation of your computer depending on the applications that you use. If you were buying ram now it would have been better to get 2 x 2 gig of memory but with what you have got it is not worthwhile changing your memory. If this was my computer I would put in the extra memory and not worry about not running in dual channel mode.

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