To answer your questions, yes, that's quite a bit of overkill. Heck, the GTX 580 is overkill even for a 1080p monitor.
For 1440x900, all you really need is something around the Radeon HD 6850 or similar. Honestly though, if you want my true recommendation, I'd say to split the money and buy both a new GPU and card to go with it. If you can afford a GTX 580, then I'm going to assume you're working with around $500. http://www.newegg.com/Product/Product.aspx?Item=N82E16824236052&cm_re=1080p-_-24-236-052-_-Product
If you buy that monitor for around $200, that leaves you $300 for your graphics budget. http://www.newegg.com/Product/Product.aspx?Item=N82E16814127575&cm_re=hd_6950-_-14-127-575-_-Product
...which can buy you a really, really nice HD 6950.
I'm using an HD 5770 right now to power my 1080p monitor, and it works just fine for most games. There's no way you'd ever need a GTX 580 for one 1440x900 monitor. You could probably power a whole wall of those monitors off of that card. The best investment you can make right now is to get a good 1080p monitor and either one of those two cards.
And as has been said, be absolutely sure your PSU can handle such cards. They draw a LOT of power.
Just my opinion.