I'm running on a Windows7 operating system with Intel E7500. But it has only 1GB of RAM of 800MHz. Now due to heavy work I need some additional memorey.
So which one should I go for, I'll get a 2GB if its ok with current 1GB otherwise a 1GB.
I'm confused if there is a problem using diffrent capacity RAM is each slot, like in first slot I put a 1GB and in other I put 2GB.
So, which one will be better?

it depends more on your motherboard than your cpu ,so what is the make and model # of the motherboard to help find the model give this a try

Lets find out what you have inside.
Use Speccy to provide details of your computer's configuration.

Download, http://209.62.107.107/speccy/download/standard  , install, and run.
After Speccy has finished gathering information, click File > Publish Snapshot.
Click Yes. A web address will be displayed.
Click Copy to Clipboard and paste it in your next post.

As mentioned by caperjack, motherboard info is very important before upgrading your ram. ALso make sure to check if your using 32 bit or 64 bit WIndows 7? The 32 bit version will only allow you to access atmost 4 gb ram, while the 64 bit version doesnot have this limitation. If possible I would recommend you go for atlast 2-3 gb of additional ram since you are doing some heavy work. Takecare.
UI Specialist,

Correct me if am wrong abt speed.

One other way to determine what kind of RAM you can get, how much, etc. is to go to one of the major vendors like crucial or kingston (also, some multi-brand vendors like 4allmemory.com) and run their memory configurator tool. For the few machines where you do not get automated responses, you can manually select by machine / MBO.

IT depends whether your motherboard is familiar with this sort of settings