Maximum PC

Howdy, I came here to ask about a build for a gamer pc two years ago and now I am back again. I would like to update my PC a bit and I need to build a PC for my Son's, both are used just for gaming. My PC runs fine except I am noticing a lower Frame Rate now and some lagging in newer games. I tend to play MMO's and FPS for the most part as do my son's. I run my games in 1920x1080 and would like to do the same on the other pc. I have $800 - $900 to spend between the two, I could squeeze more if I needed to but that involves the wife.

First my PC.. this is what I'm running currently. Just looking for any upgrades that would be best bang for my buck.

*1 x CORSAIR TX Series CMPSU-650TX 650W ATX12V / EPS12V SLI Ready
*CrossFire Ready 80 PLUS BRONZE Certified Active PFC ...
*1 x XFX HD-577A-ZNFC Radeon HD 5770 (Juniper XT) 1GB 128-bit GDDR5 PCI
Express 2.1 x16 HDCP Ready CrossFireX Support ...
*1 x Antec Three Hundred Illusion Black Steel ATX Mid Tower Computer Case
*1 x AMD Phenom II X4 965 Black Edition Deneb 3.4GHz Socket AM3 125W Quad-
Core Processor HDZ965FBGMBOX
*1 x G.SKILL 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10600) Dual
Channel Kit Desktop Memory Model F3-10600CL8D-
*1 x Western Digital WD Black WD7501AALS 750GB 7200 RPM SATA 3.0Gb/s 3.5"
Hard Drive -Bare Drive
*1 x ASRock 890GX PRO3 AM3+ AMD 890GX SATA 6Gb/s USB 3.0 HDMI ATX
AMD Motherboard

Build for Son's - Gamer Only
Looking for recommendations.

Thanks

Do you have a budget in mind for the complete build for your son and your upgrades? Off the bat the biggest items I see without doing a total platform upgrade with your system would be the GPU and the addition of a SSD. GPU wise I would recommend a Radeon 7870 or if the budget allows a Radeon 7950 specifically either a Gigabyte Windforce line card or an MSI TwinFrozr cooled card. For the SSD I would recommend a Samsung 840 120GB SSD or if you're willing to spend the cash a 240GB SSD. For the son, well I really need to have a budget in place before I start making recommendations because you can go quite crazy budget wise with a gaming build.