Should i upgrade my mac?

I have a 24" imac i bought on ebay about a year ago. I do some garageband work and light gaming. Recently when i add too many garageband tracks it starts to choke up and lag. Minecraft lags as well after around 10 minutes of play. I have a ps3 so really the only games i play on mac are COD, L4D2, Oblivion, minecraft, and sometimes WoW. My girlfirend plays Sims 3 and it runs flawlessly of course lol. Skyrim can't run on my machine even at 800x600, which scares me because i would like to play Elder Scrols Online when it is released, and if Skyrim won't run, then i'm pretty sure ESO won't run either lol. If i were to look at an upgrade, what would be my best bet? I was looking at a 2.6 i7 Mac Mini with 16gb ram and a SSD, only thing that scares me is the intel hd 4000. Any opinions?

Early 2009 24" iMac

3.06Ghz Core 2 Duo

8gb DDR3 1067Mhz Ram

1tb 5400rpm HDD

NVIDIA GeForce GT 130 512 MB

10.8.4 Mountain Lion

BTW my geekbench score is a 2900 and Cinebench OpenGL test i get 24fps and the CPU test i get 1.2 points.