I have 1gb of RAM in my pc and i want to updgrade the memory.

System
Pentium D processor 2.8ghz
Nvidia Geforce 7300 se
1gb or RAM
Vista Home Premium

I have scanned my pc and this is the results
Memory upgrades from Crucial.com - Determine My Memory Needs

what RAM shall i buy to get the best performance

If you have a good budget... I would take the 3rd option down that list
2GB kit (1GBx2), Ballistix 240-pin DIMM, DDR2 PC2-6400 upgrades for MSI (Micro Star) MS-7255 (P4M900M-L) Motherboard, CT657808 from Crucial.com

your mobo supports up to 2 gigs-- and up to DDR2 PC6400
this way you get the fastest Ram you can put on that board for $55.00 per 1 gig stick-- and it's a pair... Yes you would lose the 1 you have in there now-- but that Ballistix RAM is good stuff.
You'd be done with RAM-- then think about a Video-Card upgrade