I'm not a fan of prebuilt, but if you can get that optiplex 990 cheap its the best one, spec wise. If I had to guess, it probably has an i7 2600 and 8GB of DDR3. Not the newest or fastest hardware, but it will get the job done. Since you can get it cheap, you can spend some more cash to drop a graphics card in it to make it a capable gaming rig.
Be careful though, some prebuilt PC's, especially dells, have awkward internal layouts or weak power supplies. If you get a PC with a junk power supply you will need to purchase a new PSU before you can even think about putting a graphics card in it. If you end up with one that has a weird internal layout you might be limited on upgrade-ability.
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