What's the best hardver for PostgreSQL 8.1?

Hi!

What do you suggest for the next problem?
We have complex databases with some 100million rows (2-3million new
records per month). Our current servers are working on low resposibility
in these days, so we have to buy new hardver for database server. Some
weeks ago we started to work with PostgreSQL8.1, which solved the
problem for some months.
There are some massive, hard query execution, which are too slow (5-10
or more minutes). The parallel processing is infrequent (rarely max. 4-5
parallel query execution).
So we need high performance in query execution with medium parallel
processability.
What's your opinion what productions could help us? What is the best or
only better choice?
The budget line is about 30 000$ - 40 000$.

Regards, Atesz