For the CPU: You have two options. You can place the thermal probe underneath the cpu, meaning you would have to remove the cpu from the socket, put the thermal probe there, and manage to put the wires between the pins on the socket. Depending how wide the wires are, this might be quite inconvenient. What I did is put the thermal probe on the side of my heatsink, as close to the cpu area as possible. The read out is not quite as accurate, but it will give you a good general idea. (I like to add about 5-9 deg. celsius to the readout.) DO NOT PLACE THE PROBE BETWEEN THE HEAT SINK AND THE CPU.
For the vid card: You could place the probe anywhere on the card. I would suggest putting it as close as possible to the gpu core. I put mine on the opposite side of where the core is. I have my case probe just hanging in the case, not touching anything. Just make sure that no fan is blowing directly on it, or the readout wont be accurate.
To actually hold the thermal probe in place i used some thermal tape. Good luck!
P4 3.0 Northwood oc\'d to 3.6
Zalman Alu-Cu hsf
Asus p4p800e-dlx mobo
1 GB (2x512) pc-3200 OCZ performance series rev. 3 Ram (2-3-3-6)
ATI Radeon X800 Pro
Arctic ATI Silencer 4
Raidmax Scorpio case
Creative Labs Audigy2 ZS Gamer edition
2x74gb WD Raptor, Raid 0
NEC 8x DVD+/-RW
Windows XP pro w/ SP2
Sony 19\" LCD (SDM-HS95)