This is my first attempt at a build. I've spent a fair amount of time reading online and doing a little research but still don't really know what I'm talking about or doing.

My basic needs are the standard things: mostly surfing the web, word processing, and streaming. And playing online poker if that happens in the foreseeable future. Playing some games would be nice - I want the option - but I'm not a big gamer by any means. I think Portal 2 is probably the most intensive thing I'd play on PC in the forseeable future (that is unless Half-Life 3 if it is ever released). In any event, I'm sure I can survive without max settings. That said, I hate lag. And more power and better performance is always a good thing... at the right price.

The arbitrary budget I set was $600. I suppose this is fairly flexible. I'd like to keep it around here - or lower - but can probably be talked into improvements.

I already have a monitor, mouse, and keyboard. I think I have access to Windows 7 through school, but if not, I can buy that separately and it's not a big deal.

I will probably get a SSD and run the OS off of that in the future, but prices seem to be falling fast and I decided to save some money for now. Without further ado:

PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: Intel Core i3-2120 3.3GHz Dual-Core Processor ($116.99 @ SuperBiiz)
Motherboard: ASRock Z68 Extreme3 Gen3 ATX LGA1155 Motherboard ($114.99 @ Newegg)
Memory: Corsair XMS3 8GB (2 x 4GB) DDR3-1333 Memory ($34.99 @ NCIX US)
Hard Drive: Samsung Spinpoint F3 1TB 3.5" 7200RPM Internal Hard Drive ($109.99 @ Newegg)
Video Card: MSI Radeon HD 7770 1GB Video Card ($116.97 @ Newegg)
Case: Rosewill CHALLENGER ATX Mid Tower Case ($49.99 @ Newegg)
Power Supply: OCZ 600W ATX12V / EPS12V Power Supply ($49.99 @ Newegg)
Total: $593.91
(Prices include shipping and discounts when available.)
(Generated by PCPartPicker 2012-06-14 12:58 EDT-0400)

As I said at the start, I don't have a great sense of what I'm doing, so any advice would be appreciated. It's very possible that I have made some terrible choices here. Everything is compatible according to PCPartPicker, but if something seems off: please, please let me know. If I made a bad choice, please explain why - whether part A is under-powered and bringing everything down, part B is overpriced or far more than I need, or part C is a cheap knock off that will probably break quickly. All relevant stuff I'd like to know.

Also, one of the goals of this build is for to be easily upgradable in the future. Does this seem to accomplish that goal?

I posted this on another site and the consensus seemed to be that my motherboard was a bit high for the rest of my build, and I could probably save some money there. It's also been suggested that I get a better video card, but I guess I'll have to do a little more research on that.

Based on 2 things you said (no hard core gaming for now, and you want the build to be easily upgradeable in the future), you could also go another route.

The new Ivy Bridge CPUs have the best integrated graphics of any Intel CPU to date. Tom's Hardware tested these chips and found that the integrated GPU performed well enough for a good gaming experience.

You could pair this with a Z77 chipset motherboard (which would give you the latest bells and whistles and give you the most expansion headroom in the future), and actually do away with the discrete graphics card for the time being.

The computer would be fast, would give you an adequate gaming experience, and would be well positioned to take advantage of new hardware in the future. You could always add a graphics card later if you wanted.

A quick check at newegg shows a Biostar Z77 mb for $90 at the moment. It has SATA 6 Gb/s and USB 3.0

They also have the new Core i5 3450 for $200

That would save you about $60, and probably give you more options in the future.