is your pc a pre-built (dell, hp, etc.) or custom? if it's a pre-built, what model? if it's a custom, what psu do you have (brand/model and wattage) and what case do you have?
first i wanna make sure that you don't have some sort of slim or sff (small form factor) case that could cause size issues, and to see if your psu is decent enough to run an upgraded gfx card, not that you'd be looking at the highest end cards with your budget, but just to make sure.