Help with PSU Selection

Greetings,

First off I am good with computers but this is the first time I have dabbled in upgrading my computers other than doing RAM upgrades. So I am in a bit of a pickle here and keep getting more and more confused on selecting a new PSU for my "HP Elite e9120Y". Getting confused with Single and Dual Rail, 2 PCI 6 pin or 6 pin +2 connectors Etc.

HP in their wisdom only put in a 300w powersupply, the bare minimum I feel this system needs, and I know when cranked up gaming or watching a long movie, I can smell the PSU cooking, I believe the case is adequately cooled, (Video Card fan, Processor fan and heat sink, and large case fan) I see the CPU core sitting around 85 deg F, video card sits at around 110 deg F , case outlet around 85 deg F, but I see exhaust of the PSU reaching 150+ deg F.

My mother board has a single ATX_PSU connector 4-pin near the processor (yellow and black wires running to it) and has the 24 Pin ATX power along the outside of the board.

Here are my Computer's Specs

Mother Board: Pegatron Pegatron M2N78-LA (Violet3-GL8E)
Processor: AMD Phenom II X4 910 2.6ghz
Hard Drive: Western Digital 1TB HDD Caviar 5200rpm SATA (slow drive I know)
Video Card: ATI Radeon HD4350 512MB Video
Memory 8GB DDR3 PC3-10600 MB/sec
DVD Burner
Blue Ray Disk Player
integrated sound

My future improvements include:

New Video Card 1GB+
Another 1TB HDD to set up RAID and Faster HDD's
Faster RAM

I have been looking at the Corsair Powersupplies and like the HX series but I see they have 2 4 pin ATX connectors, would I just plug in one of them, the ones i have seen with only one connector is down around 450W may be enough but why i am asking y'all. Looking for some help and recommendations.

Thanks in advance

-DH