Well, first of all, you really don't need a hundred fps. And I don't think you'll have to spend $500 to $700. Think less. Another useful thing to know, perhaps the most important of my questions, is what fps does it run at currently?
But if you really do want a hundred fps, you should definetly get more ram, and a better graphics card. I'll tell my recommendations later.
As for the hard drive, what do you do? A hard drive will not affect gameplay much. And how much space are you using now, and do you have most of the stuff you need on the HD, or is this a fairly new computer that will have much more stuff put on it? There are a lot of hard drives and the prices and capacities differ. Also, do you need a fast hard drive, or is a medium one okay?
And what resolution, and what quality do you play at? That makes a big difference.
What speed is your ram at? How many slots do you have, and how many are used up? Do you care about ram brand?
And, do you overclock?
And also, is your graphics card PCIE or AGP?
Because of a lack of info, I cannot suggest much except that you should get a gig of ram for a $100.
If you respond to those questions, then you could get a much more accurate answer where I would suggest more parts and the exact ones at the best prices.
Edit: Do you have any other, more demanding games that you want to play? And, do you want to have this PC to still be a good gaming system in the future? How good, (I mean what games with what resolutions and qualities), and for how long do you want your system to be good?
Oh, and if you like Condition Zero, maybe you should get Source. I heard it's great and it comes with Half-Life 2 for $28 at newegg Half-Life 2 w/ Source
. Amazon also has it, I think, and it probably qualifies for free shipping.