Unless I lost my eyesight, I don't think the TS ever mentioned which PC he has? The chances of a GTX260 fitting in the average HP case are slim (slim cases you know hohohoho) and the chances of it being a Pentium 4 2.4ghz are considerable...

I've bought the Gtx 260 now.

The Gtx 260 need 550 watt minimum, and I have only 400. It works, I have tried the card. But does it brake the card, should I buy new power supply?
Someone told me this aint a problem, it just gets underpowered, and cant do its maximum effect.

Depends on the quality of the PSU you've got. If it's cheap and it's trying to supply too much power it might work for a while, then overheat/overload, fail and destroy your whole system.

If it's good quality then it MIGHT just drop out when too much power is drawn from it. Sometimes your system will just power off or the GPU might just turn off when booting up. Like when the CPU is too hot it just shuts down.

If it was me and I had a 400W PSU and my BRAND NEW Graphics card manual said I needed 550W min. I wouldn't risk using a low powered PSU and possibly wreck everything. But I am paranoid. You on the other hand might be a carefree spirit and believe everything will be okay.

400 isn't enough in my eyes.

The person that told you it "cant do its maximum effect" is misinformed. What do they think? It can only max out to 2xAA or something when it hasn't got enough juice? It's not a light bulb.