I know it's not going to get me much, but I'm wondering what can I build for 160? Which allows me to upgrade in the future? and hopefully the fastest possible?
Editted.
You can't directly mine bitcoins with any regular hardware. you need something in the ASIC category... and at 160$... well, besides the USB thingies, I can't think of any other solution. And I wouldn't recommend it because the difficulty of bitcoin mining is ridiculous. Also third gen asic miners are coming out early 2014, making anyone with under 100Ghash of power see revenues drop drastically.
At your budget, your best bet is to mine litecoins, it might be an alternative cryptocurrency but it's the second and safest after bitcoin.
160 is very little, also it's hard to say what to get as I don't know what you have or don't have.
Someone already pointed to a Radeon 5850
I agree 100%
You might also be able to find a Radeon 6950 for cheap. I bought mine for between 100$ and 150$
I would find a used crossfire or SLI motherboard (AM2, AM2+ AM3, LGA775) for future upgrade capabilities, a milk crate, lots of tie raps.
CPU could be anything, even a p4 celeron will do the job... they go for nothing.
Any buddy that likes computers might have some crappy DDR2 ram laying around... like 3 bars of 256mb ddr3-533... It's shit and useless.
PSU might be the most expensive part. Minimum 450W for 1 card, 700W for 2 cards.
All in all, you can build a decent litecoin miner with 2 cards (5770,5830,5850,6870,6850/70 for about 400$
You can start it for about 160$ if you find the right parts for the right price and upgrade it as you go...
The challenge is research and to find the right parts for the right price.
This is my third machine, I added it recently, just before litecoin value jumped above 20$
50$ used board
free celeron cpu
free ram
free ide 40gig harddrive
150$ 6950 with aftermarket cooler (Flashed to 6970 by original owner)
120$ 6950
100$ 700W psu
Milk Crate and tie raps.
Hashes about 900Khash for litecoins
All in all, I hope you have a lot of fun !