Any more than that at the same time. Read that supersite interview - the computers this is targeted at have 300-400 MHz CPUs with 64-128MB of RAM. If you try to open more than three interactive apps on a machine of that class, they're all gonna slow to a crawl anyway!
Firstly, I personally know people from the "target market" of XP Starter Edition. In Malaysia for example, it's almost impossible to get a machine of that class any more, even second-hand. I'm not sure how well received a computer costing 300 USD will be with those specifications, for it seems grossly underpowered.
Secondly, I used to own a Pentium II which was clocked at 350Mhz, and a mere 64 MB or RAM. Windows 98 performed well (even with more than 3 applications open), and Linux performed even better. But that was back in '99. It's been 5 years since then, and even in rural areas you can see better machines.
Both the resolution limitation (800x600) and the "3 programs at once" limitation seems artificial and unnecessary. I believe Microsoft is underestimating the computing requirements of their target audience. Children and young people for example, in any country, developing or otherwise, pick new things up very easily and will quickly outgrow the limitations of the "budget family PC". However, it is Microsoft's product, and they are free to attempt to market it however they see fit. It is not, in my humble opinion, the wisest of marketing strategies.