the cheapest way to buy Windows 7?

On October 22 Microsoft will offer time-limited half-price preorders of Windows 7 . In the US, this half-price offer has expired.
Under this deal, a Home Premium upgrade was $50 USD, and a Professional upgrade was $100 USD. If you missed out on this deal, Microsoft has confirmed that it is offering a "Family Pack" for Home Premium in "select markets".
The Family Pack will offer 3 Home Premium upgrade licenses for around $150 USD.

check the SHA-1 hashes for RTM

What are the SHA-1 hashes for RTM? According to a Microsoft, the SHA-1 hashes and filenames for Windows 7 English are (in sha1sum format):

; Windows 7 Retail Ultimate E English (x64)
029dccedd7691206010f84ce58343405a4da92c9 *7600.16385.090713-1255_x64fre_cliente_en-us_Retail_UltimateE-GRMCEULXFRER_EN_DVD.iso
; Windows 7 Retail Ultimate English (x64)
326327cc2ff9f05379f5058c41be6bc5e004baa7 *7600.16385.090713-1255_x64fre_client_en-us_Retail_Ultimate-GRMCULXFRER_EN_DVD.iso

; Windows 7 Retail Ultimate English (x86)
5395dc4b38f7bdb1e005ff414deedfdb16dbf610 *7600.16385.090713-1255_x86fre_client_en-us_Retail_Ultimate-GRMCULFRER_EN_DVD.iso
; Windows 7 Retail Ultimate E English (x86)
bc10f09b86dcbaf35b31b0e6fba7d006acaad28d *7600.16385.090713-1255_x86fre_cliente_en-us_Retail_UltimateE-GRMCEULFRER_EN_DVD.iso

users can use HashCheck for checking hashes.