I currently have a 4 mb. Really, this is a question you must answer for yourself. The answer depends on what you want to do with the system more than anything else. Between the two, you'll have a kickin' game machine. Of course, if you do that, you won't have any room on your hard drive to actually store programs capable of really taking advantage of your new toys.
Hard drives are a big bottleneck on modern PCs.
This fast, UDMA drive will unquestionably alter your sense of your PC's overall performance. That said, you didn't mention how much RAM you have now.