How much amps for a HD3870?

How much amps does a ATI HD3870 512MB DDR4 need on a 12V-rail?

7.5 I think.

Recommendations for the Hd3870 is a 450 watt PSU with 26 amps for a single card.

The wattage doesnt matter as long as your 12v rails have at least 26 amps. If you dont have that much of parts and you dont OC your hardware then a poweful 350w can run this card and im taking from areal expereince from my friend's pcSearchMaster

What happens if I don't have enough amps? Will my gpu just be underperforming, or mayby something else?

Usually its going to underperforming yeah because it still gets some power from the bus

Well, I just bought a Black Power LPG19. It has 580Watt and 23A on the +12V.
The guy at the store said it would definetily be enough, since I wouldn't use crossfire. But then again the same guy said I should pay for a new mobo, an extra hdd (for RAID) and a different type of RAM (800mhz instead of 667mhz) to increase my pc's speed, instead of a new gpu...
Anyway, do you think this will be enough? Oh, and I only have PCI-E1.0, so no extra power through there (if it would even use that, I don't know)

Well, I just bought a Black Power LPG19. It has 580Watt and 23A on the +12V.
The guy at the store said it would definetily be enough, since I wouldn't use crossfire. But then again the same guy said I should pay for a new mobo, an extra hdd (for RAID) and a different type of RAM (800mhz instead of 667mhz) to increase my pc's speed, instead of a new gpu...
Anyway, do you think this will be enough? Oh, and I only have PCI-E1.0, so no extra power through there (if it would even use that, I don't know)

liszt_rhapsody

First off....a lot of places tend to push products on to people that don't seem to have a lot of knowledge on computers. Though you may be very knowledgeable on computers, but if the sales person doesn't think so, then they'll be pushing.

His advice of getting another HDD just for RAID is pointless, unless you absolutely need the extra space or if you really want to go with a RAID setup. The differences in the RAM speed wouldn't even be noticed because they're so close to begin with.

A new MB probably wouldn't give too much of a better performance if you're still running off the same CPU. If you were to get a new MB, then get a newer and better processor. I suggest looking into the high end AMD 64 X2 or a higher end Intel duo core. You get better performance out of Intel, but the AMDs tend to run cooler.

With that PSU you picked up, you shouldn't have any troubles running your video card, as long as you're not trying to do any OC'ing.

My advise with PSUs; find one that'll work for you and then go up a step to something a little better. You want to be sure that your PSU has enough power to spread out through your whole system.

The Cooler Master 750W PSU I put in my computer for my dual 8800GTS 640MB cards is overkill. I could have easily used the 650W (a few less amps then what the 750 provided) to power my system. But I want to be absolutely certain I won't have any issues if I work on OC'ing my processor and/or video cards.