I'm going to check the link you sent, I hope I can give it a go.
You've got three things to consider:
1. What video card does your iMac currently have? If it's already the top end graphics card that was available for "Late 2009" 27" iMac's…then you may not have any upgrade options.
2. If you don't already have the top-end video card in your iMac…are you comfortable with what the cost will be for the higher end part? Higher end video cards (especially for Mac's) can be very expensive. You need to do some research to find this out.
3. Are you comfortable doing the upgrade yourself? If not…this is really not something you want to have done by someone else. Since the cost of the part + the cost of the labor to have it installed could cost you upwards of $500-$600 or more.
And the upgraded video card may not even perform a whole lot better. Because at the end of the day. You will still have an almost 6 year-old Late 2009 iMac. The BEST video card in a Late 2009 27" iMac…may still be a lot slower than the lowest end video card in a 2011 27" iMac.
The reason why I say this is. If you sold your current 2009 iMac…add to that the cash you would have spent to upgrade the 2009 iMac…you probably would have more than enough cash to purchase a newer 27" iMac (2010, 2011, 2012, etc.). And the newer iMac would be better in many other ways that the 2009 iMac (not just the video performance).
p.s. If this upgrade was only a $50-$100 investment…everyone would be doing it. But it's not. It's an expensive project…and the financials usually don't work out versus selling what you have…and purchasing a newer model computer.