I wouldn't buy from a business site that only offered PayPal and Google Checkout - although I am more likely to use one of those when offered along side a traditional merchant account.
It's a legitimacy issue for me. Anyone can get a PayPal and GC account with fake information. If you can't scrape a few bucks together to get a "real" merchant account, then you aren't a "real" business and I'm moving on.
Now... if you're up front about the fact that you're "just some dude" and aren't presenting yourself as a business, then I'm OK with PP/GC-only payment method. But to me, any real company should have real CC processing and if they don't I assume it's because they either can't afford it, or their credit is so bad no merchant bank will underwrite them and neither instills trust.