Contemplating about building my own dev machine

So the last time I bought a personal computer was 2011. I bought a Macbook Pro (2.53 GHz Intel Core 2 Duo Processor, 4GB Memory and YES it's still on Snow Leopard). I'm just scared that if I upgrade it to Yosemite, it will be very slow like most of the stories I've hear and read online. I was also given an opportunity at work to develop and learn about .NET. Our Windows machine (my wife's) is still on Vista and I've made it dual-boot to Ubuntu Linux. We now use it 90% in Linux and never even use the crappy Vista side of it. It's clear I'm due for an upgrade. Right now, I have 2 options, get another Mac(I also use it extensively for audio production) or a new Windows machine (so I can install Visual Studio). As of now, I cannot afford to get another Macbook Pro because we don't have enough budgeted for the one with the specs I want. As for Windows, it's always cheaper and I can always build one from scratch. I have not experienced building a PC from scratch, but it's always been on my technical bucket list. Perhaps now is the time.