System memory effect on gaming

My system specs are:
CPU: Core2Duo E8400@3Ghz
Main board: Gigabyte P35-DS3L
Memory: Geil 2*1 GB DDR2 800
VGA: ATI HD4870 DDR5 1GB
PSU: Green 485 watts

I want to put 2GB more Memory on my system. Is it okay to buy the same RAM and fill all the sluts?
How much does it affect my fps in games? How much power it will consume?

Another option is to wait and save money to buy a better mainboard and use DDR3 memories.

Plz, help me decide.

Yes, it will work.
There should be some impact on fps, but it will depend on other things like your OS (is it 64bits so as to take full advantage of 4GB of RAM?), number of applications you run while gaming (anti-virus, for instance) etc.. By and large, I would say that if your computer accesses HD to often, then you should expect greater impact.
No impact on power consumption.
About buying a new mobo, it makes no sense do buy a 775 socket mobo with DDR3, so you would have to up-grade your whole system (mobo, memory, processor).
Hope that helps.

If you're running Windows XP, I don't believe you will benefit from upgrading your 2GB of ram to 4GB. The gains are going to be very small. To put a price on it, if it's going to cost more than 50$, stick with what you have.