I just built a PC for ~$300. That was mobo, CPU (AMD A8 "APU" w/ usable integrated graphics), memory, and an SSD (I had a case with usable power supply). It scores 6.7 out of 7.9 on the Windows 7 "Experience Score Index" and is limited by the integrated graphics. The next lowest score is the "CPU" at 7.2. The $60 bottom of the barrel SSD scores the maximum 7.9.
For comparison, my multi-kilodollar laptop from 4 years ago scores 5.9, also limited by its graphics (FireGL V5700, basically an HD3650 IIRC) with the CPU and memory throughput tying at 6.6 (the drive has been updated to a ROFLOVERKILL SSD that scores only 7.7 due to the laptop only supporting 3Gb/sec SATA).
Now, these scores are far from the end-all, be-all. However, you probably don't need anything more, from the sound of it. You could probably got a little lower end, even. Using the cheapest stuff I could get at Fry's, I could have built a system for about $200 that would probably still outperform my professional/workstation replacement laptop from 4 years ago!