It is not possible to give a definitive answer to your question � computers are not like racing cars, with a single purpose (to go fast). If you take a look at computer benchmark tests carried out by magazines, you will see that some PCs are better at certain tasks than others. Some might be very good at handling graphics, while other might have excellent access time to the drives.
In your proposed application (looking at live prices), the limiting factor on the speed of such a system is likely to be your internet access speed. You will need the fastest Broadband you can access � it is likely that any Vista capable PC system will handle the broadband speed offered to it.