I would guess you are using XP 32-bit in that case, I would stick with a 1gigabyte card(for addressable space limitation reasons).
Also those specs would limit any card that can use 2gigabyes of memory.
While you can get cards like a GT 640 with 2 gigabytes of memory, it is slower then a 4870 with 512megabytes of memory. Simply put, you will find many low end card with tons of ram at a jacked up price and yes lots of users end up buying them. It even sounds good(human nature is to brag), but at the end of the day, it was NOT worth it.
Unless you are getting a much faster card(one that will leave you cpu bottlenecked allot), 2gigabytes of memory on the video card will be a waste for you.
I am not burning the idea of 2gigabyte cards, just making sure you know that in your case, it may be a waste of money.
Yeah and as Sooth1 asks, how much power(12 volt Amp rating or combined 12 volt for multi-rail power supplies.).
While it is a 280 watt power supply, video cards only use one of the many voltages the power supply has(12V and not 3.3/5/-5/-12). This means you may have 250 watts @ 12 volts or 120 watts @ 12 volts. This makes quite a difference in the system.
Yeah, by the time you got all the necessary components that wouldn't bottleneck a 2gb card you might as well have a new system all together. Most people will tell you that you don't need an i7 processor (top of the line), but many will say that you should shoot for an i5.
For graphics cards you are looking for something that can play games at high-ultra settings at 1080p. Playing games at medium settings usually isn't much better than consoles. And as Nuke said, getting a card with 2gb Vram doesn't always mean you can play the latest games at good settings. All Vram does is increase the amount of texture memory available to you, which allows for much higher resolution textures, but only certain games get close to 2gb of vram and none that I'm aware of exceed 2gb without a texture mod. I'd say for graphics cards you are going to need at least a GTX 550 or AMD equivalent to get the type of performance that you want.
Then you needs a decent amount of ram, at least 4gb, but most people will say go with 8gb. Then a PSU of at least 500w, but probably more like 650-700w to be safe. Can't skimp on the PSU and get a really cheap one either.
Then, as others have said, you have to make sure your motherboard and case can accommodate all these parts, which they probably can't. So, at the end of the day you should just buy a new PC all together. If you build it yourself you can get a killer PC for under 1k, if not you can get the same PC prebuilt for about 1300. $1300 really isn't that much for a PC that will last you at least 3-4 years without any need to upgrade.
If I ever get another gaming PC I'm going to build my own, I'm too nervous about screwing it up to build it myself, so I would pay some type of third party to put it together and test it for me, costing about $200. Even with the extra 200 bucks it STILL would be wayy cheaper than most of the name brands like Alienware.