Quick AV card upgrade question

[chandlerc]

I set up a basic HTPC in my den. I wanted to get the maximum possible output with the minimal possible input. It could have been done better, but this set-up is fine for now.

OS: Windows 7 Home Premium 64 Bit
Processor: AMD Athlon II X2 2.8GHz
GPU: NVIDIA GeForce 6150SE
LAN transfer: 10/100/1000Mbps
Audio Chipset: High Definition Audio
Memory: 4GB
Backup: 1 TB external hard drive

I use the PC 90% of the time like a network attached storage device. I'm not a gamer, so the most use it gets is streaming occasional video and audio from the web. I have an HD TV and, as you might expect, I'd like to be able to stream an HD picture.

Brand Name: LG
Model: 42LK520
Display Technology: LCD
Display Size: 42 inches
Image Aspect Ratio: 16:09
Image Contrast Ratio: 40000:1
Speaker Count: 2
Resolution: 1080p
Refresh Rate: 120

I'm a pretty handy guy, but I'm not the most tech-savvy. I can make the change myself as long as I know what to do. My tower is pretty small, so I'm not sure how much wiggle room there is to replace the graphics card with something larger.

So, what's the same size as the NVIDIA GeForce 6150SE, enables HD video output (preferably HDMI), and costs as little as possible. Just to stress, I'm only interested in watching HD video from the internet, DVDs, etc. No WoW or anything like that. If you have a suggestion, can you also point me to the appropriate drivers?

If this is the wrong place to post this question, then please relocate it.

Thanks in advance.