According to Intel, the max junction temperature is 79 degr.
so 88 is beyond recomendable.
The difference in wattage between the two celerons is ca 10w
(18w/28w), so if dkc_ace's advice doesn't help, and you are
going to by a new cooler, be sure to get one that can take ca 30w.
I have a celeron 1000@1240mhz in a very very small cabinet
and it never goes beyond 45 degr. with a standart heatsink and
a Thermaltake casefan( i know, strange mixture but it works and
it is low noise)