My recommendation for Brahms 3 & 4 Sym. in good sound:
Dresden Staatskapelle/Kurt Sanderling/Eurodisc (BMG Classics) - economical choice as well as this is a complete set of 1 - 4 on 3 disks for a paltry sum of $13 and performances are stupendous on all 4 sym, only negative being Sym. 3 is split across disk 2 and 3.
Tchaikovsky PC #1:
Sviatoslav Richter/Karel Ancerl/Czech PO/Supraphon - decent sound in mono, but no other performance comes close, certainly not Argerich. BTW, because CPO toiled under the iron curtain for so long,few westerners know their sound, but rest assured, they are a world class orchestra.
Emil Gilels/Fritz Reiner/CSO/RCA Living Stereo - if you must have a stereo recording, this is the one to get. Argerich/Abbado/BPO/DG is also supposed to be quite good, but haven't heard it.
Tchaikovsky PC 2 & 3:
Mikhail Pletnev/Vladimir Fedoseyev/Philharmonia/Virgin - not to everyone's taste, but I like it quite a bit. Perhaps not romantic enough for some taste.
Igor Zhukov/BMG/Melodiya (74321496122) 2CDs
Tchaikovsky Complete music for Piano and Orchestra -
haven't heard this myself, but people whose opinions I respect tell me Zhukov's PC 2 is the definitive version.