Question.

I know a guy who wants to convert his rig into a gaming rig but he has no idea of his specs or anything.
His rig is a pre-built dell, model: optiplex 755. This is the manual- http://www.dell.com/downloads/global/products/optix/en/opti_755_techspecs.pdf

He basically has 500$ to spend and is looking to play modern games (BF4, etc) at any settings and playable frame rates (around 30 is enough). He has a 1080p monitor and needs no peripherals. He is ordering from Amazon.com (US)

Thanks you in advance for your replies, I know this may be a tedious question.

I forgot, he instaled 6GB of RAM to that build

cpu-z and gpu-z freeware software will tell you what you got inside the rig with allot of details.
But it might be a proprietary machine and wont play nice with other hardware if everything is integrated onto the motherboard: video,sound, memory... ( if its that old.) Grab a screwdriver and open that sucker up and look.

In it's day, that was a top of the line office system with a cost of $1183. Today you can buy them all day long for between $139 to $234.
As for upgrading it......He'd be better off taking his $500 and building a new rig.

Grab an AMD CPU and GPU for starters. Should save you some money. Look out for CPU+MB deals. What kind of ram is that? DDR2? DDR3? Look for Ram included deals too if the ram is too old. You can build a decent system for that much.

Lets see.

Budget $500
-$90 Windoz. He can't use the old Dell Windoz.
Old case, no cooling for newer hardware -$50
Op has plans for BF4 on $460, he needs a full system.

Op needs to come up with $300 more to play on a 1080p monitor at a minimum with basic playable decent framerates.

$800 to start.

Buy a used GPU if you're lucky on an auction on Ebay. Hell or buy another persons system for that budget. I am sure there's people throwing out their rigs that will do 20-30 FPS for BF3/BF4 with low-mid settings.