If you don't NEED to have the latest and greatest, X3 is still a very good product with lots to offer. It ran/runs very stable on my Windows 7 64-bit powered by an i7-3770.
If you plan to keep it as an isolated audio system for a few years there's nothing wrong with a setup like that. For speed recommendations I would suggest:
1. An SSD for the OS drive and main program. 120 GB should suffice if you keep it clean, but if you can spring for it and you have a lot of samples go for 500GB or something. Speeds up loading times incredibly. Samsung EVO840 got good reviews but I'm sure there's always another "latest and greatest" here too.
2. Get a large 7200RPM audio drive. I personally like the Western Digital caviar black Raid Edition 4.
3. Make sure you have true USB2 ports as well as the new USB3 ones. Not all audio interfaces like the USB3 ones. This is what Alex is referring to with the new standard coming.
4. Your interface determines minimum latency even more than your CPU speed, depending on what you're doing. I don't know what interface you have but it may be worth sinking some money into that. RME is often recommended on here and definitely has top notch performance.
Personally I only have 8GB of RAM and I've never run into its limits yet. Disk streaming in modern samplers (Kontakt etc.) makes sure RAM use remains under control. But I suppose with current prices and looking to the future, 16GB isn't all that outrageous.