Welcome to the PcGuide forums.
$250-$300 is tight for a mobo, CPU, and video card. We need to know what your current system specs are as well.
What type of ram and how much do you currently have? SDRAM, DDR, or RDRAM?
What's the rating on your current PSU? Wattage and the rating on the 12v+ rail.
What type of case do you have? Micro ATX, Mid-tower ATX?
If you have a pre-made system it would help to have the make and model number.
It can be done if you don't need anything else as the recommended system specs are not that great for Guild Wars.
Recommended System Specs:
* Windows XP/2000/ME/98
* Pentium III 1GHz or equivalent
* 512 MB RAM
* ATI Radeon 9000 or GeForce 4 Ti Series with 64MB of video memory
* 500 MB free hard drive space
But you may be better off figuring out what is wrong with your current system depending on what you have. Then saving up more cash for a better core upgrade later. But again this all depends on what your current system is and can it be fixed.
Also what else are you going to do with the system? Do you want to play newer games etc.?
What steps have you gone through to troublshoot the system? What symptoms are you having? If needed you can have a local shop test the mobo and CPU for a small fee.
If you can answer these question we can go from there.