I think you would be safe with most of the low-end GT7xx cards as technically they're just GT6xx cards (The GTX745 is one exception).

If you want to stay period correct for a C2Q, you might want to go with a GTX280 or similar.
Otherwise, I see people indeed recommend the GTX7xx series a lot. The GT730 however would be a bit on the weak side (much slower than a GTX280 for example) and personally, I wouldn't spend anything near $30 on one. Look at the GTX750Ti instead, the cheaper ones can be had for about 40-50 EUR over here and will run circles around any GT730.

But what about a more modern middle-end card? Would it work and give an higher boost? Cause I have the same situation and was thinking if using an old DDR3/G41 board as a base for a "modern low-res game machine".

I tried a Geforce GT 1030 on a G41 build about a year ago. I had no end of issues with it locking up and blue screening on both Windows 7 & Windows 10. The card worked fine in my newer computers. That is why I asked about going with a GT 7xx card instead. I think a GTX 2xx card may be a better choice at this point for both compatibility and price.

After a bit of deliberation, I am going to take different approach. I am currently running ArcaOS in VM. It would be nice to run it on actual hardware. This current build would be perfect for that use and I can just use the onboard video for which ArcaOS would work much better.