I currently have a socket 754 AMD Sempron 3100+ system. The problem that I am running into is that I don't have a good enough graphics card to play semi modern games(Red Alert 3 being the game of interest). I have about $100 extra right now and by looking online I can upgrade to a new motherboard which has a PCIe 16x slot and still use the rest of my older hardware. My question is this. By just upgrading the video card without upgrading everything else am I going to experience a bottleneck with the rest of my hardware and will it be a significant bottleneck? The rest of the system consists of 700 w psu 2 gb DDR ram 2 Sata HDDs 300 gb & 500 gb HP DVD RW So what do you guys think?

Tough call. If you could save up another $100 or so and get some sort of dual-core processor I imagine it would vastly help things as well.