Some games can use 4 cores like COD4, supreme commander, Crysis etc
However those games are not coded very well for 4 cores, thats why quad core will not make any big difference compared to a similar clocked dual core.
Single core, nowadays are outdated. There is no reason to buy single core, because even the budget processors are dual-core nowadays. Intel released celeron dual core E1200 which is based on core 2 architecture but it just have lower cache (512KB). So even celeron nowadays are dual core
Newegg.com - Intel Celeron E1200 1.6GHz 512KB L2 Cache LGA 775 65W Dual-Core Processor - CPUs / Processors