Three kinds of design ability are needed: product design (what does it do?); software design (how does it do it?); and graphic design (what does the site look like?). Somehow between them the founders have to cover the first two. Graphic designers can be hired, though it's better if the founders can do that too.
The ideal configuration would of course be a group where all the founders had all these skills. Haven't seen that yet, though.
My ideal candidate, were I doing the funding, would be a group consisting of two hackers and one non-hacker. Maybe I'm biased though because that's what we have.
If I recall correctly, PG did say that every team should ideally have a developer, a designer, and someone good at both to be a "sweeper."