If you are not well up on computers, one of the big problems when it comes to upgrading is knowing exactly what you can stick in there and have it work properly.
A useful tool to have in these circumstances is this, a little utility called CPU-Z:
http://www.cpuid.com/cpuz.php
You can download it for free from the above link (although naturally, they appreciate donations). It analyzes your system and reports on exactly what you've got in there, (motherboard type, RAM speeds etc etc) Which is particularly useful when it comes to upgrading your RAM, as you'll need to get the same type of RAM again if you want your old and newly-purchased RAM to work compatibly alongside one another. Different RAM brands can sometimes prove incompatible even with all the same specifications, so choose the best match you possibly can (look for an identity sticker or name printed on your currently installed RAM (unplug your computer before poking around inside it, you don't want to electrocute yourself, do you? And earth yourself before touching any sensitive components inside your PC to prevent damaging them with static electricity - you can earth yourself by touching something metal like a radiator in your house for example).
When it comes to graphics cards, it can get confusing to know which is which and what one does what, and since most people tend to either strenuously defend either NVidia, or ATI (depending on their personal experiences rather than vast technical expertise), a lot of opinions can be somewhat partisan. While advice is okay from a forum, you're far better off setting aside an hour or so and doing a little research, like this:
Armed with your info gained from CPU-Z, you will know exactly what your motherboard is, with that info you can do a search on it and get an exact spec sheet on what it can handle. With this info you can then search for compatible products with the certainty that what you are looking at buying will work okay. A good time-saving way to do this is by going to a site such as the one linked below (or any other one you prefer for that matter). In addition to listing lots of products in either price order, or name or whatever, each product gets a little spec description, so you can search for stuff in your preferred price range, and then check out that it tallies spec-wise with your motherboard:
http://www.microdirect.co.uk/
It's not rocket science, just requires a little methodical research. Good luck with it
