Intel Corporation makes both the Pentium and Celeron branded processors. A Celeron is a lower grade intel, whereas a Pentium is better grade. Generally you'd want to go with pentium if all other playing fields were equal, but in this case you have more variables.
Clock speed effects performance, as do all the other components that make up your computer. I'm assuming the celeron, with it's higher clock speed is newer, and therefore has better/newer components around it, so i'd use that box for gaming. I make this assumption w/o knowing all your other components tho. If the P4 box has a wicked new graphics card and awesome RAM, and the celeron has integrated graphics and bargin bin RAM, i'd go with the P4 box.
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