How to check/replace video card

How can I check if my video card needs to be replaced? I am experiencing an intermittent problem where sometimes my computer only boots up on the 2nd monitor, and lately it sometimes comes on with both monitors covered with strange squiggles. And sometimes neither monitor will come on. I am thinking this may be the video card but I don't want to replace if it's really the motherboard or ram or something else.

Here's my machine specs:
- HP Pavilion Elite m9300t PC with TV/PVR
- Genuine Windows Vista Home Premium with Service Pack 1 (64-bit)
- Intel(R) Core(TM) 2 Quad processor Q6700 (2.66GHz)
- 8GB DDR2-800MHz dual channel SDRAM (4x2048)
- 1GB NVIDIA GeForce 9800GT, 2 DVI (VGA and HDMI adapters)
- 750GB 7200 rpm SATA 3Gb/s hard drive
- 802.11 a/b/g/n Wireless LAN card
- LightScribe 16X max. DVD+/-R/RW SuperMulti drive
- 15-in-1 memory card reader, 2 USB, 1394, video, audio
- ATSC-NTSC TV tuner with PVR, FM tuner, remote
- Sound Blaster X-Fi XtremeGamer

Another question is, if I do need to replace the video card, how do I select the correct card for my machine if I can't find the exact same cards?

Thank you for any help with this.
Dave

Things to check, if you haven't already:

1. Check to make sure all fans are working properly. You could try something like SpeedFan to see what temperatures are detected also. Make sure there is no dust buildup on components.

2. Remove and reinsert the video card. Check for corrosion on the slot connector contacts. If no improvement, try a different slot on the motherboard, assuming one is available.

I assume that you have the stock HP power supply in that computer. Post the wattage rating.

Also, try uninstalling and reinstalling the graphics drivers. Use updated ones if there are any available.