Which of these 3 selections is the best option, don't take price in to consideration.
 
Crucial Ballistix Tracer 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model BL2KIT25664AR804 - Retail  
 
http://www.newegg.com/product/prod [...] 6820148180
 
OCZ Reaper HPC Edition 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model OCZ2RPR800C44GK - Retail
http://www.newegg.com/Product/Prod [...] 6820227267
 
(2) OCZ 2GB ( 2 x 1GB ) PC2-8000 1000MHz 240-pin DDR2 Memory - OCZ2TA1000VX22GK
http://www.buy.com/retail/product. [...] caid=15891

The performance differences between these aren't likely to be noticeable in real life use.  If I had a windowed case, I'd think the Crucial Ballistix Tracer would look "interesting."  Otherwise I'd choose #2, the OCZ Reaper.  I'm not good at ignoring price, and the DDR2-1000 will not offer enough of a performance difference to justify its price.  Depending on your mobo, any DDR2-800 is less likely to have compatibility issues.

Some motherboards have issues running 4 sticks of memory, and speed increases that small won't be noticeable. Between the 2 4GB kits the difference is minor, so go with the Reaper, since it is the cheapest and should work just as well as the Ballistix.