A bit of googling and I've found out a bit more about Darcys in the late 17th and 18th centuries.

From that site on Whittington:
http://freepages.genealogy.rootsweb.com/~spire/whittington/
~the Earl (afterwards Duke) of Devonshire, the Earl of Derby (afterwards Duke of Leeds), Lord Delamere, and Mr. John Darcy, eldest son of the Earl of Holderness, assembled to concert measures for effecting the revolution of 1688. ... A jubilee, or centenary, of the great event, took place here in 1788, when a procession was formed, consisting of upwards of fifty gentlemen's carriages, many clubs, with their bands of music, and a large assemblage of the most distinguished families of the county'

This Mr Darcy was therefore instrumental in the Glorious Revolution of 1688, which established the Whigs as the dominant power in England. It ties in with his family connection to Fitzwilliam, the leading Whig magnate in JA's time.

From a site on Holderness in New Hampshire
http://www.nhes.state.nh.us/elmi/htmlprofiles/pdfs/holderness.pdf
`Holderness was named in 1751 for Robert Darcy, fourth Earl of
Holderness, ambassador to Venice and minister at The Hague under King George III. He opposed the king's policy toward the colonies, and became a close friend of Governor Wentworth in efforts to promote friendly trade relations abroad.'

More Whig connections, since the Whigs were opposed to Lord North's actions in America (note another Wentworth, btw)

From the register of marriages in York Minster
http://www.genuki.org.uk/big/eng/YKS/Misc/Transcriptions/ARY/YorkMinsterMarriages4.html
`1748, Oct. 3. John Murray, of the city of York, Esqr, & Dame Bridget Wentworth,* of the city & diocess of York. (Lic.)
* Bridget, only daughter of Sir Ralph Milbanke, Bart., of Halnaby, by his first wife Elizabeth Darcy, eldest sinter of Robert, earl of Holderness. -he married, first, in March, 1731, Sir Butler-Cavendish Wentworth, Bart., of Howsham, who died without issue in 1741 ; and, secondly (ut supra), John Murray, Esq. Lady Murray died in 1772.'

Yet more notable Whig names

From a site about horse breeding
http://www.highflyer.supanet.com/investigation.htm
`When Charles II returned to London on May 29, 1660, after nine years in exile, one of his first acts was to appoint James Darcy the elder Master of the Royal Stud at Tutbury in Staffordshire. He was appointed on June 6, 1660 and remained in office until his death in 1673'

Mr Darcy was instrumental in establishing the English Thoroughbred horse: there are a fair number of sites about this.

My thoughts
This seems to confirm the theory that JA chose the name `Fitzwilliam Darcy' to get a resonance going among her contemporary readers to think `Whig' (unless she wanted them to think `noble horse breeder'). Why? Some suggestions:

-- The Whigs tended to be favourably disposed towards the new rich merchants and industrialists: hence Mr Darcy's friendship with Mr Bingley, whose father made his money in trade.

-- This mercantile and industrial connection goes a long way to explain the puzzle of Mr Darcy's situation: what is someone as rich as he, with a beautiful house, doing with no title? Though Pemberley itself has been in the family for generations, it's likely that a good part of that ten thousand a year comes, not from land, but from investment in factories, shipping ventures, canals, coal mines and so on.

-- The leading families in country towns tended to be Tory, as JA's family were, so there would be another source of anxiety for Mr Darcy in mixing with them as he is supposed to do. `Darcy, on the contrary, had seen a collection of people in whom there was little beauty and no fashion, for none of whom he had felt the smallest interest, and from none received either attention or pleasure.' The word interest is ambiguous: it could mean both `he wasn't interested in them' and `they were not of his political interest'.