How much minimum memory is required in a laptop to give good performance and with what frequency. e.g. 1GB, 2GB... and frequency like 667mhz, 800mhz or more than this?

With a PC that has 667 mhz or even 800 mhz, a PC with 256 mb of ram or even 512 of ram should suffice...

What processor is it running, and what os?

what processor is it running, and what os?

Intel core 2 duo T5800: windows vista

... then I'm assuming you have something like 2 ghz? By 800 mhz, I'm assuming you're talking about fsb.

Usually, for that sort of a processor, I assume 3 - 4 gb ram should do it.

Is this a good configuration?

Intel core 2duo T5800(2.0ghz,2MB L2cache, 800mhz fsb)
RAM: 2GB memory upgradable to 8 GBtype: DDR2 667mhz
HDD: 160GB speed: 5400 rpm

Yeah, it's a decent PC.

You "may" want to upgrade the ram to 3 or 4 gb, depending on what you're going to use the computer for. If it's just internet, work, and other things, you should be fine. If you're using it for gaming and hardcore graphics, then 3 or 4.

Just to get back to the original question: the frequency of the RAM you want to add is dictated by the motherbaord type in your PC. In a laptop RAM is often hard to add/replace by the user, it depends on the make/model.