If you could provide links and/or prices it would help people make suggestions. I can almost promise this 1TB Samsung F3
for $50 shipped after promo code EMCKDJF27 is cheaper than that Western Digital, and it's quite a bit faster than their 5400 rpm Green HDD's. And obviously it has roughly 50% more space.
As others have suggested, you're probably better off using the 2x2GB set only, and leaving the 1GB stick as backup. Like they said, you'd force the system to run in single channel if you added the extra stick, and it's at least 2-3% faster in dual channel, some would say even better than that. Additionally, anytime you mix RAM, you have to run all of it at the same voltage, speeds, and timings. You didn't specify the modules you're using, but I would assume they have at least slightly different voltages and/or timings given that they are different sizes and therefore use different IC's. It's not a given, but likely. All RAM will have to run at the speed/timings of the weakest link, and if the weakest link is the 1GB module, you'll hurt performance even more.
The GTX 460 1GB is a nice choice. My GTX 465 is the rough equivalent, and I run all my games at 1920x1200 and full textures (4x AA, 16x AF) with no issues. Faster cards out there, but the 460 should be good enough for the next year or so and is a great value.
Which Seasonic PSU are you using? They're generally very high quality, but every brand has a wide variety of models. And what are you paying for it? You can probably get a better price
erformance ratio from a different brand, but I completely understand you not wanting to compromise on the PSU.