I have 1.25 gig RAM, (tho' looking to add an extra 1 gig this month) and the following works fine for me, 30 Leagues from 18 Countries
Argentina Top league
Australia Top League
Belgium Top league
Brazil Top League
China Top League
England Blue Sq North / South
France League 2
Germany League 2
Holland Top League
Italy League 2
Mexico Top League
Portugal Top League
Scotland Scot's 3rd Div
South Africa Top League
South Korea Top League
Spain League 2
Turkey Top League
USA Top League
When playing my "long term Career" I have them all as playable and start as unemployed and only apply for jobs in the lowest possible(either Blue Sq North / South or Scot's Div 3), Then I am set for a realistic World tour.
My other 2 main games are as Aberdeen and Man U boss, but with those as I know i will remain at each club until I retire or am sacked I load the other league as view only. I managed to get to around 2030 in FM2007 with my full set up and compressed game file with no probs and around 2010 with Aberdeen & Man U, I find the career lasts longer as it is harder to get repeated success with different teams and after you have won everything about 5 times with AFC and MUFC the game loses it's appeal. I have noticed the game processes a lot slower than on prev versions (I have BETA 2 patch loaded) but hopefully the ful patch and an extra gig of RAM will fix this