Which disk to upgrade in Mac Mini Server 2009

Hi,

We have a Mac Mini server late 2009. One drive is used for the OS and the other store files which are accessed across our network.

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Model Identifier: Macmini3,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.53 GHz
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A while ago we replaced one of the internal drives with a bigger Hard drive - a Hitachi HTS547575A9E384 750GB. Since doing so access the files stored on the machine seems slower. It is only about 2 thirds full.

We are now considering upgrading both the RAMM to 8GB (currently 4GB) and the Hitachi Hard drive we've put in which seems to be struggling.

Will this improve things?

Will replacing the HD the server OS on improve things?

Thanks!

Two options I recon;

Same capacity but faster or smaller capacity and WAY faster. We'd have to do some trimming - currently using 440GB - and it's more expensive.

Existing_

Hitachi Travelstar 5K750 HTS547575A9E384
— 750gb
- 300 MBps drive transfer rate
- 5400 rpm / 8mb Cache

Suggested_

Western Digital 750GB WD Black ---- $87.99
- 750gb capacity
- 600 MBps drive transfer rate
- 7200 rpm / 16mb Cache

OWC Mercury Electra solid state drive ---- $374.99
- 480gb capacity
- 600 MBps drive transfer rate
- rpm NOT applicable

Plus the RAMM upgrade too of course.

Thoughts?