I'm sure that the i5 is faster than the core 2 duo, but there's always something new just around the corner. As long as my computer is fast enough for my needs then I don't worry too much about the latest technology. If you want to keep up with the cutting edge then it is going to cost you a lot, whether you run a Mac or a PC.
The broadband connection is a separate issue. All modern computers are plenty fast enough to cope with a broadband connection; the bottleneck is always the connection, not the processor, hard disk, memory, or any other factor. As a (non-perfect) analogy, think of a car. Most modern cars can easily do 80mph or more. If you can only manage to drive at 40mph because a road is congested with traffic, then it doesn't matter if you have a cheap hatchback or a Ferrari - they'll both only be able to do 40mph.
A broadband connection is like a permanently congested road. If you're lucky you may be able to do 40mph, if not you may only be able to do 20. With networks there are the equivalent of freeways, that let you travel faster, but they cost extra. If you want to drive at 70mph you're going to have to change your road.