Honestly, wait. Don't build/upgrade a rig till closer to FFXIV if you can.
I'd be pretty surprised if your current machine won't run FFXI as it's quite old and doesn't take much. The longer you can wait, the lower the prices will be. If you need a machine before then, give me a budget and what preferences you have (AMD vs Nvidia) and I'll provide you with a couple setups if I can. Are you planning on building it yourself or buying a pre-built OEM machine like a Dell, or similar?