Also if I'm not mistaken or not suppose to mix brands on memory right?
It's generally not a good idea but won't exactly lead to disaster.
Are you sure your memory is discontinued? Check a few more websites, I found some Ballistix 2x1GB 800 on Newegg:
Newegg.com - Crucial Ballistix Tracer 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model BL2KIT12864AL80A - Desktop Memory
It might not be the exact same model but it shouldn't give you any trouble.
If you'd rather be sure, I'd buy a 2x2GB set from Corsair or OCZ (etc.). 800 or 1066, there's a marginal difference between the two, you're unlikely to notice any boost in performance from an increase in speed.
Keep in mind that if the XP you're running is 32-bit, you won't see all of that RAM; you'll see about 3.5GB minus your VRAM.