Radeon cards questions

Hi, i'm not exactly a huge computer whizz with hardware + mechanics, so i'd like to start off by asking would another 3870(512mb) be any use to my machine or would it bottleneck? (or any programs to test this?)

I have a 2.8 dual core processor, 2xDDR2 OCZ 1GB Ram (6400's), (1xAsus EAH3870 - 512mb, 777Core, 1126memory), Asrock ConRoeXfire-eSATA2 motherboard, 700 Watt OCZ PSU.

Something i'd like to add is that i get bluescreens (but they don't show it's just a soft-reset, with microsoft informing me it was a blue-screen) after exiting CoD2 and running 3DMark06 on the CPU test with the desert/mountain range. (not seen anything else blue-screen me yet). - As far as i know i have the latest drivers and updates so i very much doubt it's that before you ask

And secondly, would you recommend a HD4850 instead, or possibly a 4870 (and then ofc would it actually run properly?)
I mainly play Warcraft myself (not too huge on graphics that, and i believe it doesn't have X-fire compatability) but other games i have are; BF 2142, CoD4, Crysis, CoH + CoD2 - I've read that the 48's would be more beneficial than crossfiring 2x3870's due to AA but i never really have it on anyways, at most x2 and i normally run at 1280*1024 seem as i only have a 19inch screen

Thanks for reading and hope you can answer my questions, if you need any more information just ask and if a tool is required to obtain it, please recommend one

Typing up from the card... "Intel Pentium D processor 915, LGA7755 Pkg 2.80Ghz, 2x2 L2 Cache, UP only"
oh and my Mobo says "1xPCI-E x16, 1x AGIE (PCIE x4), 1xPCIE x1, 3xPCI slots"... i'd assume cross-firing wouldn't be amazing from x16 down to x4??

getting 2 gpu really going to suck because one of your pcie slots is 1/4 its max potential so i advise you to get the ATI 4850 instead of crossfiring BUT most importantly upgrade cpu since its a socket LGA 775 you can get one of the newer intel core 2 duos but you MIGHT need to flash your BIOS to get it to recognize it