Faced a similar dilema myself. I forgot to pick up a tube of thermal grease for my tbird when I bought the case at the local shop, so I was all prepared to run it w/o it. Then I realized the $5-6 in gas I'd spend to run to my second job, pick up a tube, and come back plus my time was in no way worth $155. So I was a patient little PC Builder, went and got some, and the system ROCKS now.
Waiting may be the hardest part, but when building a pc, sometimes its the most important step.