Once you get over a Gig the difference is slight. I'm told that Battlefield 2 prefers 2 Gig but thats the only game I know of that demands so much memory, every other game that I have seen so far will run on either of the systems you describe so the choice is really yours. Specialist gamer companies like Alienware for example use standard branded components for the most part and provide in my opinion at least a higher quality item, better case, better power supply better cooling etc but this does come at a price and unless you are a hard core gamer, which you say you're not I see little point.
The thing to do is get a balanced set of components, there is no point having the fastest processor available if you only run with 256mb ram or expect it to play the latest games with a GeForce 6200. Same goes with having the fastest graphics card available and using it with a Duron 800 or whatever.
See what Gladiator are offering by all means but try to compare same for same if you can to get an idea of how the prices vary. If you like Dell and have had good service then by all means stay with them, they produce PCs at a price that I cannot even get the components for.