Sorry but I have to say somthing about all this,
IMO a Desktop computer is better than a Laptop, the reason why is cos Laptops die way faster than Desktop, they can be upgraded as much as a Desktop, and if you need a better Graphocs crad, in a Laptop you'll find then to cos way more than a Desktop.
People have said that you need to make sure you get a lot of RAM for the memory, this is true, the more ram the better, but then comes the problem, if your running an 32 Bit operating system, cos on a 32 bit OS can only handel $GB of RAM, (well more like 3.6GB of RAM) and will not acept any more so getting over $GB of RAM in a 32 Bit operating system is just a wast of money.
If you get a computer with a 64 Bit operating system, then you can have up to 128GB of RAM, so making sure that you have this in mind whan you get a computer.
But RAM means nothing with out a good proceser, as a Intel Celeron, proceser, is the worst one you can get, so look out for somthing along the lines of a Core 2 Duo, with a Quad core, this means the proceser should last you for some years.
But then there is the mainboard, (mother board) if this is not a good one then there is no point spending all that money on getting the best proceser and RAM, when the mainboard BUS wont handle the flow af data as fast as it's being sent, so again you need to be looking at this as well.
As for HDD (Hard Disk Drives) A SSD (Solid-state drive) is, well just like a USB pen drive, but one that you can't remove, and is inside your computer like the HDD, SSD cost more, as they are some what new to the market, so getting a HDD drive may be more cost efective, I'm in the UK and I can get a 2TB (terabyte which as 1024 GB total for a 2 TB drive is 2048 GB) for £69.99, I did a quick look online and found a SSD will cost £62 for only 80GB so you can see a SSD drive may not be what you are looking for.
The best thing to do is let us know what contry you are in, and the limit you have to spend, also whether you looking for a Laptop or a Desktop.
As for the Mac vs PC, this is just what you feel like you will want, there good and bad point to both of them, so this is up to you, but I will say if you only go for a Mac, cos they are somewhat virus free, then I feel you are making a big mistake, as Mac don't get hit with them that much cos 70% of people run Windows, and theres no point making a virus for a system thats not being used by a lot of people, thus you wont get the max damage you can get if you code it for windows.
Anyway thats my thought on the whold point, feel free to as any more question