Allow me to give you a vew words of advice. When you say, "Building a computer slowly" I'm assuming you are refering to buying parts a little at a time as you can afford them, right?
As you get the parts, build.
If this is the case, please take my advice and don't do it. Instead save money a little at a time. Put $100 here - $200 here - $250 here... ETC... Wait until you have saved up at least 3/4 of the cost of the total build. Then buy the bulk of the main parts. CPU/MOBO/HDD/MEMORY/
Why? Technology is changing so fast. Every month something new comes out to replace or upgrade what is currently out. Not only does this effect prices, but it also effects your build. If you buy a CPU and MOBO now and this PC won't be done for another 3-4 months anyway, wait... That same CPU that you buy now for say, "$250" in 4 months might be "$175" and the MOBO is now $80 and it was $120. Now that you just saved over a hundred dollars you can upgrade to better RAM or HDD or whatever, you get the idea.
Set a budget: Start saving your money. While you are saving it price out all the parts. Write down that total figure. Continue to price out those same parts over the whole time you are saving. I can tell you with 100% certainty that you are going to see the total cost of that PC continue to go down. So my point is why buy now when you can wait and save and buy all the parts at the same time and save money. If you can't use the PC now, why buy parts that you know will go down in price day by day?
Well, if you are a hobbiest and have tons of cash, do whatever you want.