How much better is new (Late 2013) iMac than (Late 2009) iMac?

I have a maxed out (Late 09) iMac with 1TB HDD - 8GB Ram - 512MB HD4850 Graphics Card and 2.8GHz i7.

It's at Apple getting the Graphics Card replaced (for the 2nd time).

I only moderately game. Diablo 3, WoW, Marvel Heroes (when it's EVENTUALLY released for Mac) nothing too graphics intensive.

I'd like to know if the (next to the) bottom of the line iMac (not with Integrated Graphics) would be an upgrade for me...

I'd splurge for the 16GB RAM (since it's not user upgradable) and also either the 1TB Fusion or 256MB Flash. I think that would be enough until I could find a USB 3.0, 3or4TB external for $100-150.

My desk is really small and I feel like I'm right on top of my iMac now, being 27" so going diwn in size to 21.5" isn't really a problem.

My question really is - I'm basically at $2050 for the 21.5" ($1499 iMac + 200 RAM + 200 Flash + 150 External)

But it's only a little more (relatively speaking - $500 is $500 but over 3 - 5 years is it really that much?) to get the 27" with 8GB RAM (I could wait and replace it myself) and the 4GB Graphics Card and still the 256 Flash.

I mean the base 1GB Graphics Card in the 21.5" is already DOUBLE what I have now but I don't know how it performs. and I'm sure the Flash Drive will make things run better but I just don't want to spend 2 Grand + and feel like dropping back to an i5 is going slower and I didn't get an upgrade at all.

Yeah. That's what I was afraid of. Diablo 3 killed my 1st HD4850 in March. I had it replaced. Then, a few days ago, I decided to give WoW another chance after stopping for about a year and a half or two.

I thought "I will just go with the free Starter Edition" I'll just make a few new guys, get them to level 20 and if I feel like I still miss out on playing, I'll re-sign up.

Downloaded it. Opened it. Created a new Pandaren Mage. Got to the customize page - chose skin color, hair color, etc. BOOM! Frozen!

Ok let me try this again. Open, customize, enter world, BOOM! Little grey squares everywhere. Frozen. Ok I'll try one more time. Restart. 5 giant grey lines mown my screen on top of EVERYTHING.

Are you kidding me? Dead Graphics Card AGAIN???

Apple was nice and said they would cover the cost of replacing it if it is just the graphics card because it's the exact same problem that I had back in March even though the warrantee on the card is only 90 days. 90 Days - that's all they give?

I'm still waiting to hear back. I'm torn between hoping it's just the card that's the problem so They'll replace it free & hoping that's it's something bigger that's wrong like the logic board so I can tell them NOT to fix it so I can get a new one but I really don't want to spend all that money on a new 27" with the 780m.