I'm guessing you're thinking about the $320 Revo as you mention the dual cores.
Straight up I'd be willing to bet for most tasks the Athlon XP is faster than the Atom 330. For some multitasking the Atom might be a little smoother. Either way neither system is going to be doing things the other can't unless it's something the graphics chip brings to the table.
The better purchase here would be the Dell Zino HD. Upgrade it's processor to the cheapest dual core offered and then add WiFi and whatever else you might see fit such as more hard drive space and you've actually got a legitimate upgrade for the same price as the Acer Revo.