Depends on the quality of the center. Some drivers can even emulate a center speaker from the fronts if necessary. Without knowing what sound card you have, I don't know if you have that capability. I use my PC for everything (music, movies, games), so having a high-quality center is very important for me.
4.1 just for games would probably be okay. Heck, I ran 4.1 for everything for years, just using a phantom. I'm there again now, because my new sound card (Asus Xonar) has revealed to me that my center speaker isn't that great, and I have to try some others until I find what suits my ears best. In your case, the center matches the fronts exactly, so you won't wind up with a tonal mismatch. If the price difference isn't huge, and you plan on keeping this for a long time, go with the 5.1- it'll be more future-proof. Better to have and not need, than need and not have.
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