According to VG chartz, after selling 350,000 on its first day, Dissidia has solid over 500,000 cumulatively in the whole week. This is better than Crisis core, which sold about 480,000 in its first week.
VG Chartz.com - Video Game Sales Charts - Nintendo, Sony, Microsoft, Wii, PS3, Xbox360, DS, PSP, PC
Impressive stuff. Its now the third best selling PSP game in Japan this year. It'll probably beat second place in terms of overall sales though, but probably not by the end of 2008.