Careful! – don't mix up the GST/HST credit (or the new Ontario Sales Tax Credit) with the Ontario Sales Tax Transition Benefit (OSTTB). Each is a distinct program. The first two are ongoing and called credits, whereas the OSTTB is a "transition benefit" or "rebate". I know, it's confusing! :-/
By the amounts you've mentioned, the $300 and $1000, I understand you to be referring to the OSTTB, and not the credit programs. One more thing to clear up: OSTTB payments are not yearly, but one-time. There will be three payments, only: June 2010, December 2010, and June 2011. Then it's over.
To answer your question: Widows or widowers, in general, are classified as single individuals for the purposes of the OSTTB, except where the spouse died after December 31, 2009.
Refer to the Ministry of Revenue's FAQ, Who is considered to be a single individual? Excerpt:
You are considered to be a single
individual, if you are single,
separated, divorced, or widowed, and
you are 18 years of age or older and
do not live with any dependent
children under the age of 18.
If you are married or in a common-law
relationship with any dependent
children under the age of 18, and your
spouse passes away after December 31,
2009, or you are a single parent
living with one dependent child under
the age of 18, and your child passes
away after December 31, 2009, the
family rate will continue to apply for
your payments following the death.