I didn't read all the replies here, so excuse me if this has been said already.  I run a Scarlett 2i4 on Win7 Home Premium with i5 processor, 8 GB RAM, and SSD for system and SATA drive for audio, documents, etc.  It's my only PC that I also run Adobe apps and internet on.  No need for a dedicated PC these days, IMO.  Everything runs great.
 
Given that you have Waves and NI stuff, I'd say go with Sonar Studio rather than Producer.  The base system is the same, just Producer comes with more instrument and effect plug-ins.  I've run X2 Producer for a while now but now I also have NI Komplete, so my next upgrade is going to be Studio.
 
Welcome back to the fold!

Some of this is parroting previous posts, so please forgive the repetition.
 
https://www.cpubenchmark.net/ is a good resource for looking at computer components if building a machine. This machine has never been reformatted/reimaged since I installed an SSD in it 3+ years ago now. I use this machine for everything under the sun, including high-end 3D gaming, am always connected to the internet, and install/uninstall programs often (I currently have 240 programs installed).
 
CPU-wise: Intel i7 Sandy/Ivy bridge models (ones with K at the end, e.g. i7-3770K) are more than sufficient for X3.
Graphics: X3 is not 3D-intensive, so a decent card will suffice (most cards do 2D more than adequately).
RAM: I initially got 8GB on this machine knowing I could upgrade if needed, and have never needed (ironic).
Interface: The 2in Focusrite models are very adequate for a solo performer who overdubs.
(I personally prefer desktops over laptops simply due to upgradability and ergonomics).
 
The only work that strains this machine is hard-core video editing (Adobe Premiere Pro), which can put heavy loads on both CPU and RAM, depending on what is being done. Anything audio related has not phased this machine (and my SSD and graphics card are overkill for DAW usage). But also take this with a grain of salt, as work flow plays into this as well (excessive plugins or CPU-intensive plugins can eat up CPU/RAM if not paid attention to).