cognitoo, I agree whole heartedly with you regarding your concern about the 460W PSU. 460W does seem too low to me too especially with a GTX 260.
Please refer to:
http://www.tomshardware.com/forum/253543-33-power-supply-question-gtx260
According to information from the above link, the GTX 260 needs about 160 Watts just for the card. A certain card manufacturer's website recommends a minimum of 500 Watts for the system using a GTX 260.
Personally, I prefer a PSU overkill rather than deal with the power fluctuations created during operation when a marginal (or barely adequate) PSU is used in a system.
Instead of buying a stock (or customized) computer from HP (good reputation), I would suggest that you look into building your own. If you decide to build your own, then you can pick the parts from different manufacturers after reading customer reviews.
Probably the biggest advantage of building your own is that you will have a clean license of Windows OS (recommend Win 7, 64-bit) without all the junk that companies like HP, Dell, Compaq, etc., add to the OS before shipping. Before building my own computer, I was plagued by problem after problem on a daily basis on my fairly new Dell desktop. This was what prompted me to bite the bullet, read up on new builds, and after 2 months and several hours of reading, I went ahead and built my own i-7 system. After I had it running in 2 days, I read up on overclocking the CPU and did that. Fun way to gain knowledge. Another advantage is the fact that you have the flexibility to upgrade/add components when you wish to do so.
Thanks to the members on this and other forums for guidance. There is a wealth of information right here at Tom's Hardware!
Good luck cognitoo!