First, do some research and build your own system. Buying a machine from a vendor like Dell is an enormous rip-off and comes without the personal touch a custom-built system has.
Secondly, most OSes take advantage now of multiple processors/cores. Also, most current games take advantage of multi-core CPUs. For example, BioShock, Crysis, UT3, and CoD4 all take advantage of multi-core processes.
Also, since the Core 2 Quad is 64-bit, you may want to get XP64 or Vista 64 in order to take example of 64-bit processing, which allows you to address more memory (I think the limit is 2 GB for 32-bit?) and enables certain extra features. For example, Far Cry's 64-bit mode enables enhanced graphical features.
However, the problem with 64-bit versions of Windows is that you may experience driver incompatibilities as well as trouble running older games and some programs.