Nice rig there Freaky ! Your Lanboy already has 2 x 120mm fans which is good case cooling.
Cooling:
For the CPU : The options are ASUS Starice (Available in India) and Thermalright XP-90 or XP-120 (XP-120 is better) which you will have to get from abroad.
nV Silencer is the best cooling available for your 6800GT, but I don't think that is available here too.
If you want you can watercool your rig. Some watercooling solutions like Thermaltake Aquarius II are available here, but others need to be bought abroad.
Oc'ing
First, set the Hypertransport Bus to 4x or 800Mhz. Next locate the PCI/PCIe freqeuncy settings (not sure where they are, just search, I got a A8V) and set it to 33.33/66.66 or something like that. These are very important steps, not doing this will screw your PCI devices and your 6800GT.
Then look for the memory settings. Set the divider to 5:3 (or let it remain 1:1 if you are confident of overclocking your RAM). Next, under the CPU settings tab, Increase Vcore (CPU voltage) to about 1.475v, and slowly push the FSB by 5-10 Mhz. After every 20-25Mhz your may have to increase the voltage by 0.025v. People with a A8N-E will be able to help you better.
For the GPU : Firstly use 71.89 drivers for the card. Then open the NVidia control panel and under Clock/Frequency settings, select manual overclocking in Performance 3D. Click Detect Optimal Frequencies. Do this when your system is absolutely idle. This gives you a rough idea on how far the card will OC. Then from your stock speed, increase by 5-10 Mhz core and 10-12 Mhz memory till your reach the Optimal Frequencies. Then test the card in some games and 3D Mark 2005. Then you can increase by 3-5 Mhz and subsequently test.
Note patience is important, hope I helped you