Either a HD 6970 or HD 7870, used of course and on Craigslist. I got my 2x HD 6970's for $220 and do 1000 kh/s. That's cheaper than buying a HD 7950 for $300 to $450 that does only 550 to 700 kh/s.

Mining Litecoin isn't as profitable as mining other Scrypt coins. Mine a profitable Scrypt coin then convert to Litecoin and/or Bitcoin later. Litecoin is a scrypt coin itself too.