The video requirements are not high enough to warrant 9800 Pros or better. My CPU ensures that I'll get the fast frame rates I want, regardless of video card. The video hardware of the Dreamcast is not good enough to require a high end card, but that 200MHz CPU is powerful and will take a decent CPU to get good speeds - this is emulation and not PC gaming which is basically the opposite requirement-wise. I agree with Jabaturbo.
On-Board sound is fine for emulation, as well as cheap audio cards. Audigy 2s are not necessary at all. The money is best spent on the CPU.