Did the Radeon HD 2400 help? or are you otherwise okay, now?
I do everything as low $$ as possible (retired. no money, honey).
A couple comments. We need to know what operating system does not work, and what OS did on the older system?
I think your motherboard should run the Intensity.
You may have a problem with a fight between your motherboard audio drivers and the Intensity (Decklink) audio drivers. Uninstall the MB audio drivers, then install the Intensity to see if it helps.
Since you asked about lowend computer recommendations:
I had a AMD Sempron 754 on an ECS GF6100-M754 Motherboard – v1.0 with NVIDIA GeForce 6100, an older nvidia chipset than yours, but it ran the Intensity Pro just fine, and the serial ata controller was fast in Raid 0. But it was not sufficient for capturing the very high disk data rates required by the Intensity because it only had 2 sata ports (and I needed 4 ports for good enough Raid 0 speed, although there are 2 x drive external sata products, like the Cavalry CADA001SA2 1TB dual-500GB drive box x 2 boxes == 4 drives — fast!) and the ECS nvidia graphics controller was not what I needed since I wanted hardware video decoding. I got the ATI HD 2600 Pro HDCP, sorta like yours.
Replaced that motherboard with an ABIT which had 10 sata connectors (but the sata Raid 0 stuff always crashed on complex, long file procedures). I used an Intel E2160 dual core 1.8GHz which I ran at 3.0GHz. See, I’m cheap.
I finally ended up using my old Gigabyte GA-G33M-DS2R motherboard with a quad Q6600 (stock 2.4GHz running at 3.1GHz) because the cpu was already mounted and in a case. I could use the E2160 @ 3.0GHz and it would be more than enough horsepower for the Infinity. When I capture uncompressed HDMI 1080i59.97 there is only ~7% cpu load, and some of that processing is running the disks. Works wonderfully and no Raid 0 crashes. That MB has only six sata connectors but I have two dual-drive external boxes. Hooked my other four drives to the other four connectors. Note: it takes masses of storage for video.
Enough rambling, jerry.