Hmm, the graphics card will definitely be the biggest thing holding you back there. You should definitely try to overclock that CPU also. On a 300w PSU you wont be able to run much there. Maybe get a $50 or so PSU (500-600) watts and a 6870 or 6850/560. What processor do you have out of curiousity?
AMD Phenom 9500
Are there any tools you would recommend to overclock my stuff safely and easily? I'm pretty new to all this.