With a budget of 1500 (of course i can go less expensive too), all I have chosen so far is:
$150 keyboard/mouse, likely razer #### this is an absolutely enormous waste - trust me on this. People who buy expensive mice/keyboards are generally rubbish at gaming and think that buy spending loads of $$$ they'll get better - which is absolute nonsense. Go for a nice $50-60 Logitech media keyboard and mouse setup. Any more than this is a huge waste. Personally I've got this => http://now-order-cheap.co.uk/?id=B000T5KPK6 with an old logi mouse, and I am pretty insane at whatever games I play. </modesty> :)
$225 lian li case with 750-850W corsair PS ####not too fond of Lian Li at all - you don't need a full tower case remember, so I would definitely recommend the CM Storm Enforcer - it's a fantastic case with amazing looks, stupendous airflow, and a really well designed interior - it was a dream to build my rig with it, and it will comfortably contain the largest GPUs. http://www.cmstorm.com/en/products/chassis/enforcer/
You'll then need a PSU, and I find the OCZ ModXstream 700W to be fantastically priced for the job, and it's modular so you can have your cables nicely organized :)
$100 1tb hard drive - I would think carefully here - do you need so much space? If so, that's fine, but if not, spend the money on a faster drive. Make sure whatever you're getting is SATA 3 (the latest), and at least 7200rpm. I cut down from 1tb to 750gb just so I could get a 64mb cache drive too. I would recommend this: http://www.dabs.com/products/western-digital-caviar-black-750gb-7200rpm-sata-6gb-s-64mb-7DJ6.html
Need suggestions for:
graphics card - Now, think about what games you will play / what movies you might watch etc. You said you want something upgradeable - I would go for the 6850/70 or the 6950 (someone correctly stated that you can unlock the 6950 to a 6970, but it can be risky, and you must get the right brand (the 6850 can't turn into a 6870)
The 6850 will max out most games comfortably, but will struggle on Crysis and BF3, *only* managing high settings in 1080p rather than very high :) - the 6950 will max Crysis and BF3 comfortably. You may want to go for a 68xx and Crossfire a year or so later if necessary.
You definitely want the i5-2500K - there is no doubt about it (you will hear a lot of "the i7-2600K is better.. blah de blah... multi-threading..."... NO it's not, for normal use. Unless you are some ultra keen programmer who is performing multi-threading tasks on a daily basis, you will see *no* difference. Get the i5-2500K, OC it when it starts becoming slow (relative to the market). As for the motherboard, you need a P67 chipset (remember this) with two PCI x16 slots. I went for this: http://www.ebuyer.com/265918-gigabyte-ga-p67x-ud3-p67-socket-1155-7-1-channel-hd-audio-atx-motherboard-ga-p67x-ud3-b3
solid state hard drive - optional, might be out of budget, what size/worth it? Naaa :) you might want to invest in a small one for your OS, but later on when they get a bit cheaper - atm not worth it really.
memory/ram - how much? - get a solid 8GB RAM (2 x 4gb) and make sure it's compatible with the motherboard you're after (if you go for the one I said, then get corsair vengeance 1600mhz, and OC it using XMP in the mobo BIOS (your motherboard will tell you how to do this, don't worry!)
cd/dvd drive - anything I need to consider? - Defo go for a bluray drive - they're only about $30 more, and it's totally worth it. I got a Lite-on Bluray reader (you don't need a writer as it's far more expensive) and it plays blurays just fine, along with DVDs and CDs.
monitor - what is a nice standard size that will work well with a graphics card, 21"/23"??? I got a HANNS-G 23" LED monitor, which is an absolute beauty. It cost around £120 (how much in dollars i can't think of right now) so it's a real bargain. It supports DVI-D and VGA (you'll want to connect via DVI-D of course :D )
soundcard - i don't think i need one, could just get a usb dac/amp...my dac has coax&optical, maybe a motherboard will output this? I'm new to sound and stuff, but I don't have a sound card, and I listen with my Ety's just fine :)
Anything else I'm missing besides software? - Um... yes - use the website pcityourself.com - it's an absolutely mindboggingly fantastic site for beginners, and it was like a dream come true for me when I built my first rig.
Lastly, when you have come up with your final rig, or if you need help, post on Tom's Hardware forums - they will give awesome advice. Also message me when you have your spec sheet ready.
As for where to buy from, try NewEgg, Ebuyer, etc. Go for 3 different online stores, find the cheapest price for each component, and then order - this way you won't pay too much for shipping by buying through loads of sites, but you will save around $100 by comparing.
Thanks, any and all help is appreciated!