I recently bought a Gigabyte GTS 250 but the thing is, it looks kind of small compared to the ones i have seen online. It only requires one 6pin connector aswell and appears to be shorter then my 8800GT. Am i getting the same performance from the Standard GTS 250's? I regret buying it now.
Gigabyte is known to have smaller boards than the competition. Performance wise your card is on level with other GTS250 cards. There is a slight difference between cards like the overclocked Asus DK you linked to which is going to perform a little bit better but you can probably overclock your Gigabyte card to the same level with out issues. So the answer is;
You did not loose any performance by buying the Gigabyte one
Go for XFX, especially one with a double lifetime warranty. So that when it comes time to upgrade a couple of years from now or even next year, the buyer gets the warranty as long as they register it after the purchase. I have an XFX GTS 250 ann XFX GTX 250 and couldn't b happier as both cards have been problem free.
Your 8800GT is based on the G92 chip at 65nm, while the GTS 250 is based on the G92b chip at 55nm. Newer G92b chips have better yields than earlier ones so they require less power to run at the same speeds.