What's the best OS for my computer??

Hi there! I was wondering if you could give me your answers on this matter.

I want to know what assembly/parts, OS and software (if aplicable) would you use, based on the following specs and requirements:

Parts Available

CPU:
1xIntel P IV 3,2 GHz, FSB 800 MHz, LGA 775
1xIntel Pentium Dual CPU E2140 @ 1,60 GHz, FSB 800MHz, LGA 775
1xIntel Pentium D CPU 3,2GHz, FSB 800MHz, LGA 775, L2 2MB (x2)

Motherboards:
--> Packard Bell RC415 (1xPCIe, 3xPCI, 4xUSB, 1x LAN, 7.1 Audio, onboard video up to 256 MB shared, 2x RAM Slot)

--> Asus P5GC MX/1333 (1xPCIe, 1xPCIe 16x, 2x PCI, 1xLAN, 4xUSB, Audio, onboard video, 2x Ram slot (note: good overclocking capabilities, straight from the bios) )

--> Foxconn P4M900-8237A (1xPCIe, 1xPCIe 16x, 2xPCI, 1xLAN, 4xUSB, 7.1 Audio, onboard video, 2x Ram slot)

Memory:
4x Kingston 512MB PC2-5300 DDR2-667
1x 1GB PC2-4300 DDR2-533
1x Transcend 2GB PC2-6400 DDR2-800

Hard Drives:
1x Seagate Barracuda 3,5'' 500GB 7200 SATA II
1x Seagate Barracuda 3,5'' 250GB 7200 SATA II
1x Seagate Barracuda 3,5'' 120GB 7200 IDE
1x HDD 3,5'' 160GB 7200 IDE
1x Samsung HDD 2,5'' 160GB SATA II

Grafic Cards:
1xNVIDIA GeForce 8400GS 512MB 400MHz (Core @ 567MHz)
2xNVIDIA GeForce 8600GT 256MB @700MHz (Core @ 540MHz)


Requisites:
#1 -->1 PC for "Server" purposes (deals with pdf and doc/excel documents library organised in folders, so no special software to organise them is required), should be able to be easily accessed remotely to this specific partition, the other partitions will be destinated to recieve diferent types of files. Needs to run ocasionaly some windows software.
Should also be able to open and edit .doc(x)/.xls(x)/.pdf(x) files for some office work and also some web surfing.

#2 --> The other machine is for gamming purposes to be runned on a 48'' LCD mostly for gamming, ocasional web surfing and downloads. And should be able to run from older games like warcraft III till some more recent ones.


So, I'll be witing for your answers, and will be looking forward to hear your advices/opinion about this matter. Also If aplicable refer wether you would or wouldn't do overclocking and why/how.


Thank you all, and Best Regards!!