configuring new server / many slow disks?

Hi performers,

after following this list for a while, I try to configure a database  
server with a limited budget.
Planned are 2 databases
- archiveopteryx -  http://www.archiveopteryx.org/sql-schema.html
- ERDB - https://www.chaos1.de/svn-public/repos/network-tools/ERDB/ 
trunk/database/ERD.pdf

In peak times I expect something like
- 50 inserts
- 20 updates
- 200 selects
per second.

Current configuration is:
- Tyan S3992G3NR
- 2 x Opteron 2212 (2GHz)
- 8 GB RAM (DDR2-667)
- ARC-1261ML with 1GB and BBU
- 16 Seagate ST3250820NS (250GB, 7200 rpm, 8GB, with perpendicular  
recording)

1 raid 1 for OS (FreeBSD) and WAL
1 raid 0 with 7 raid 1 for tablespace

Can I expect similar performance as 5 drives at 10k rpm (same costs)?
Should I revert to a single-CPU to prevent from oscillating cache  
updates between CPUS?
Anybody experience about NUMA stuff with FreeBSD?

Do you have any suggestions to enhance the configuration, staying at  
cost level?
Please advice.

Axel
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Axel Rau, ☀Frankfurt , Germany                       +49 69 9514 18 0