Upgrade Mac mini's graphic card I am doing well with my Mac, but I don't want to buy another computer for gaming, then I'm thinking on upgrading it.

Searching in the web, I've seen that there is no way to upgrade the Mac mini's internal card, but anyone have experienced or knows if it works well with an external graphics card? 

Do you think it's worth? Suggestion for an external graphics card?

Mac mini's model is mid-2011 with 8 GB RAM.
If your Mac mini has a Thunderbolt port (which the mid-2011 models do), you should be able to add an external graphics card when those become available. Until then, there are no external graphics adapters for Macs except the DisplayLink USB adapters that @bmike mentioned, but those won't work for gaming.

An external graphics card is usually PCI-Express x16 and Thunderbolt only carries an x4 signal, so you won't get full speed out of that, but it's better than integrated (or maybe even just old) graphics. (Heck, many, if not most, PC motherboards only run x8 signals to attached x16 graphics cards, so they're not using the full bandwidth anyway.)