Upgrading a system

There are many ways you can upgrade a system, to name a few, adding more memory, upgrading to a higher CPU or adding a second graphics card.

Bare in mind that adding a second graphics card in SLI or crossfire does not double the performance due to driver scaling. It also does not double the amount of graphics memory availible, this is because of the way it works, generally two cards render in SFR (split frame rendering). This is were each card gets 50% of the frame to render so each card does not get half the work since it renders roughly twice as fast.

There are also a limited number of upgrades possible before you need to change motherboard, you can only upgrade to a cpu on the same socket your are using, and you can't install DDR3 in a DDR2 or below motherboard. You may also need to upgrade powersupply to power new components. You also need to remember that there is a chance you will need to upgrade case to house larger or different sized components aswell as to provide cooling better cooling.

So, if you are a gamer and you are running an old graphics card it is definantly worth upgrading to a newer one. The card you choice should depend on the type of gaming you are into, for example if you just play world of warcraft you won't need anything more than a nvidea GTX550Ti or a AMD HD6870. However if you like to play games like crysis and more graphically demanding games you need to consider cards like the GTX580 and the HD7970. You can also think about adding a second grapics card in SLI or crossfire to increase performance, but remember that a single faster card is better than two slower cards.
In order to determine what is holding your system back you can run benchmarks like PC mark or even the standard windows index, which ever gets the lowest score you should think about upgrading. For example if you have a high end graphics card like a GTX570 and your running an old pentium CPU the chances are that the CPU is holding the system back and you should upgrade to a newer processor.

If you are video rendering and you need to upgrade you should look at your specs and decide what is too low, for example, if you are only running 2GB of memory you should think about going too 8GB or possibly even more. If your software is CPU based and your budget allows it upgrading to a 6 core CPU would be worth the money, on the other hand if the software you use supports CUDA rendering you should think about getting an new nvidia graphics card with as many CUDA cores as possible, for a example a GTX580 has 512 CUDA cores were as a GTX590 has 1024 since it is a dual GPU card.