Nobody's suggesting a top end machine. You didn't mention what CPU you actually have, but I'd be willing to bet a $60 Sempron 64 and a $55 Socket 754 PCI-E motherboard would be a significant upgrade to what you currently have, and it would be about the same price as the motherboard you're talking about getting. Same price, better CPU.
You haven't given many details, but it's hard to envision a scenario in which it is remotely practical to buy the motherboard you're talking about. You're essentially saying that you want to spend $100 on a throw-away motherboard then buy a video card that would be bottlenecked by your old CPU.