Will 2Gb Ram works with 1gb in parallel

It will work. But specification of both the rams must be same.

What is the frequency of the existing Ram? Try to purchase another ram of exact same frequency of the existing one for better compatibility. However, currently it is very hard to find DDR2 ram any other than 800 MHz frequency. So if your existing ram is not of 800 MHz, get the 800 MHz ram although there might be some incompatibility issues. Normally if Rams of different frequencies are plugged they run on the frequency of the lower one.

Most probably it will work but as cilus said, try to find the Ram with same frequency as of your current one.

Would work but you might get freezes and BSODs.
Its recommended to get same speced RAM

Will work fine if the frequencies are same.

its getting repetitive...same questions on RAM FSB etc..

@ OP -your mobo supports 2Gb ram stick per slot ( 4GB Total ) and max ram speed support is 667 Mhz - so assuming your current ram is 667 Mhz but you might have to get 800Mhz ram module anyway ( cilus has already told you this ) and it might work with your existing ram module if the latency/timing of 800Mhz module @ 667 Mhz speed is same with your existing 667 Mhz module or else just purchase 2x 22GB 800 Mhz module - they will cost you around 2.4k

ram with same speed will work perfect as i am using it and answering with the same system.but if they are different then it will work with the fequency of lower ones as said by cilus and topgear.