I think the most important question is this:
WHY? What do you want to do with this computer?
The i5 chips are great CPUs, but if you want a modern day gaming machine I would suggest an AMD Phenom II X4 or the i5-750 (or i5-760) for CPU. Quad cores are becoming more and more prevalent, and games are really starting to take hold of multicore use.
As ct1615 mentioned, the resolution of your monitor will play a very large factor in what video card we recommend to you. Also, whether this is a gaming computer, or an office use computer will play a very large factor as well. For example, with a modern 1680x1050 or 1920x1080 monitor resolution, most video cards under $100 aren't the best option. However, going $120 to $150 can make a huge difference in over all performance gaming wise.
Again, it all depends on what you intend to do with this computer. The higher resolution your monitor is, the better a video card you need. The better the video card, often times the better power supply you need to feed it.