Building a Midrange gaming computer, need advice.

I am looking to piece together an upgrade or buy a barebones computer. Mine is 5 years old, cobbled together by myself and friends. I work on servers, but I love to game at home. I've heard plenty about how gaming computers can't be anything less than probably twice what I offer as a budget. I've made systems run far longer than they should have, using outdated hardware and enjoyably playing games on them since making my old x386 run pentium class games back in the day. Here only in my 30s, I'm starting to feel a bit fossilized around all these young'ins blaring Pandora on their I-pads. All I'm asking here is for kind suggestions and a bit of help for my needs:

- A powerful enough computer to play the games I want to play now and a few years from now.
- A graphics card not over $300. As much as I prefer the high end cards, I am a man of humble means.  
- I prefer Planetside or Guild Wars over the BF3 crowd, but I'd like the ability to know that at least I could give it a try.
- I'd prefer to not trip over high definition blades of grass going 4 fps, but I don't require fully realistic cinematic quality.
- I would like to purchase either the full system, or the parts I need to fill out my case to upgrade what I have... under $800.

This is what I'm looking at right now (so, you can tell me if I'm wasting my time).

- Intel Core i5-3450 Ivy Bridge 3.1GHz and a basic motherboard.

I've heard of the shiny i7s, but I'm also balancing the graphics card in this budget and would prefer to even out the bottlenecks I can afford to. The basic mb comes from my terrible habit of running my systems into the ground before upgrading. By the time I've come to the point of upgrading, everything new wouldn't work with what I have. Case in point, my current dilemma.

- GeForce GTX 550 ti

I'm told this is "good enough" by a few people. I've seen the benchmarks and frankly, they all seem to blend together in the $200-300 range in terms of performance. I also somewhat struggle with all the many variations of the same card in different models and modes. Smoke and mirrors. Just give me mark 1, 2 and 3 so I can pick the 2 and be done with it.

Memory doesn't seem to be very expensive nowadays and I'm curious if it matters anymore what kind I get so long as its compatible and matches. I could say the same for power supplies, though there's always the warning of "you get as good as you pay" in that category. I'm thinking in the ballpark of 8Gb mem (two 4s) and a 650W around $70.

I will probably also be snagging a copy of Windows 7 for this as I'm sure my copy of Windows XP won't remotely handle this level of tech. Also heard Window 8 pops out this year, though so I'm wondering whether its worth waiting for (or paying for). I honestly wish we could get back to a time when the Operating system actually operated the system and I wasn't wasting resources left and right on so many built-in features. /shrug.

If more information or clarification is necessary, just ask. Your recommendations and educated experience will be graciously appreciated.