Networking - adding pci-e network adapter cards

Hi,
I'm having two computers with marvell onboard Gb lan controllers- P5B Deluxe and P6X58D-E. The P5B never showed 1000Gbps speeds to select in the configure option untill i installed the latest drivers from marvell. Both are connected via a cat6 patch cord. On my P5B after the connection is recognized, it always shows up a pop-up box saying the network connection does not have full connectivity and that 1Gbps speed is not available. Why is this?

1) Is it because I'm using a patch cord, instead of a crossover cable. But does'nt the onboard LAN ports have auto-sensing for the type of connection.

2) Due to this problem, I decided to buy two Gbit pci-e network adapter cards for the two computers. Is this move needed?
Will it solve my problem. If so, More importantly, will it improve the network performance considerably?

I may buy a Gigabit ethernet switch later, with the addition of one more computer.
So, A) is it better to use the onboard lan ports and route it through a multi-port gigabit ethernet switch.

Or B) Buy a single port pci-e Gbit ethernet adapter card for two of them, and a dual-port Gbit adapter for the main system.

Which would give me flawless performance?

[ Note; I do professional audio work involving routing of many ( at least 60 stereo tracks at 48Khz - 24Bit ) between the two computers in real time ]

Thanx in advance