Is it risky to replace an iphone battery if you are carefull and use the instructional video's on youtube?

Also, on amazon,ebay, ect ect i see these 1600 mAH batteries... They are like 6$. How safe are these? i mean, is a higher mAH harmfull for an iphone 3G?

Apple would charge me 90$, waaaaaaay to much in my eyes

higher mAH is storage capacity not usage. they should be fine if you get them from a respectable source, and if you do it yourself and follow instructions you should be fine. Tad bit of advice, remember to follow are anti-static procedures when handling electrical devices.