Performance - moving from oracle to postgresql

Hi:
I'm beginning the push at our company to look at running
postgreSQL in production here. We have a dual CPU 2.8 GHZ Xeon
Box running oracle. Typical CPU load runs between 20% and 90%.
Raw DB size is about 200GB. We hit the disk at roughly 15MB/s
read volume and 3MB/s write.
At any given time we have from 2 to 70 sessions running
on the instance. Sessions often persist for 24 hours or more.

Total Free Free
Mb Mb %

IDXS_EXT10 2000 290 14.5
DATA_EXT100 10000 3200 32
SYSTEM 220 95.2 43.3
IDXS_EXT100 20000 9600 48
DATA_EXT10 6000 2990 49.8
UNDOB 4000 2561.1 64
TEMP 8000 5802.9 72.5
DATA_LOB_EXT20 2000 1560 78
IDXS_EXT1 500 401 80.2
DATA_EXT1 4000 3758 94
Total Instance 56720 30258.2 53.3

There are some immediate questions from our engineers about performance

"- Oracle has one particular performance enhancement that Postgres is
missing. If you do a select that returns 100,000 rows in a given order,
and all you want are rows 99101 to 99200, then Oracle can do that very
efficiently. With Postgres, it has to read the first 99200 rows and
then discard the first 99100. But... If we really want to look at
performance, then we ought to put together a set of benchmarks of some
typical tasks."

Is this accurate:
accoring to
http://www.postgresql.org/docs/8.0/interactive/queries-limit.html
-- " The rows skipped by an OFFSET clause still have to be computed
inside the server; therefore a large OFFSET can be inefficient."

What are the key performance areas I should be looking at?
Where is psql not appropriate to replace Oracle?

Thanks in advance, apologies if this occurs as spam, please send
Replies to me off-list.