Panthip Plaza on the New Petchburi Rd is worth a look; 5 floors dedicated to IT. Whilst most of the software on sale there isn't legit, some of the major hardware manufacturers have official outlets inside, and there are also a couple of IT superstores there too. That's where I went for my Mac.

As far as an ISP goes, I use True broadband (http://www.truecorp.co.th/eng/products/online.jsp) which has rarely let me down. I can't comment on any of the other providers as I've no experience with them. One problem with the net generally in Thailand however is that you'll never see speeds as advertised by the ISPs. The problem is Thailand's lack of bandwidth capacity on the international cables.