GBH is 'indictable only', so it must go to Crown court.
The maximum penalty for racially aggravated Section 20 GBH is 7 years imprisonment (rather than the 5 years which normally applies to Section 20 offences).
The table of guideline sentences, which will be used by the judge, can be viewed on Page 15 of this document:
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
('Page 15' refers to the page number actually printed in the document. Adobe Reader will recognise it as 'Page 19 of 36').
The sentences there apply to first time offenders who've been convicted after a trial. Your guilty plea and your previous convictions will roughly cancel each other out but the racial aggravation will push the sentence towards the upper end of the sentencing range shown in the right hand column. So, for an offence which wasn't premeditated and didn't involve a weapon, you might be looking at a sentence of 8 or 9 months imprisonment, unless the assault resulted in grave injuries or disfigurement (in which case, a sentence of around 3 years might be more likely).
Chris