re: minorities - As Jerome mentioned in the comments it is illegal to collect stats on race or ethnicity so all such figures are rather unreliable. That said, I believe the estimates for the number of non whites in France (Arabs, blacks, asians) are on the order of 10-12%. IIRC there is only one or zero in the National Assembly.  There is currently a debate about whether to institute affirmative action, which the French call 'positive discrimination.'  Most people are against it and the arguments cross party lines. Perhaps the most important advocate of affirmative action is Nicolas Sarkozy, Minister of Interior, head of the ruling party, enemy of Chirac, and probably next presidential candidate of the right in 2007.

re: Senate - Should be mentioned that the senate is much weaker than the National Assembly. Also it is not elected through normal elections. Only elected officials get to vote - meaning lots of small town mayors.

re: centralization - While it isn't the completely top-down system that it was until the early eighties, as Jerome mentions it is still very centralized. One aspect of that is that there is a national police force controlled by the ministry of interior and the prefects. Prefects are a little like governors of the departments except that they are appointed by the minister of interior rather than being elected.