HELP - HTPC - Gaming Build - Power\GPU question - Antec ISK 310-150

About two months ago I completed my first homebuilt PC (with the help of this forum) and couldn't be happier.

It was a simple of PC. I used the following parts:

Intel Core i5 3570K @ 3.40GHz
ASRock H77M-ITX Motherboard
8 GB Dual-Channel DDR3
1TB Hard Drive
Coolmaster ATX tower

I re-used my old Dell 530 as a HTPC by adding a GeForce GT 430 video card and installing Linux with XBMC. It is amazing but the old Dell takes up too much room in my den.

I was lucky enough to be gifted the following new PC parts:

Intel BOXDZ77GA70K Motherboard
Intel Core i5 2500k CPU
16 GB Kingston Technology HyperX Blue Ram

I want to use all the parts I have and upgrade my office PC and build a new HTPC for my den using the Antec ISK 310-150 case. This case comes with an external 150 watt PSU.

Here is the new breakdown for each PC:

OFFICE PC - This will be used mainly to search the internet, MS office products.
Intel BOXDZ77GA70K Motherboard
Intel Core i5 3570K @ 3.40GHz
16 GB Kingston Technology HyperX Blue Ram
1 TB Hard Drive
Coolmaster ATX tower

HTPC - This will be used to watch ripped Bluray movies (1080p), HD content streamed from the internet and gaming if possible.
Antec ISK 310-150 with 150 watt PSU
ASRock H77M-ITX Motherboard
Intel Core i5 2500k CPU
8 GB Dual-Channel DDR3
Unknown Hard Drive
Unknown Video Card


What video card will work with this build? The PSU is only 150 watts so I know I am limited. I don’t expect to play games at the highest resolution setting but I am using a 60inch TV and I don’t want it to look like crap. I will be compare the graphics to my PS3.
Will undervolting and underclocking reduce power usage of CPU help save power for the GPU?

Thanks in advance for your help.