with an onboard graphics card it will use your system memory(ram) as video memory, and this will make the memory available to windows and applications less, depending on how much physical memory you have you get some pretty good onboard ones that you can bump up to 512MB if you want, but then i would suggest at least 3GB of ram in your system. a completely new graphics card is the better option in my opinion. and as i mentioned one that supports hardware shading will greatly improve your preview window.
i cant help you with prices, im in south africa and stuff here is waaaayyyyyyyyy expensive!