You can crossfire with the 1gb but it will only use 512mb of that memory (1GB total, 512 each card), you see with crossfire both cards need to share the same attributes otherwise it would be difficult to spread the work across evenly.
just get another 512mb HIS 4850, To be safe tell us what psu you have or send us a link and we will check it out.
Newegg.com - HIS H485FN512P Radeon HD 4850 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card - Desktop Graphics / Video Cards
or from here
SuperBiiz.com - HIS ATI Radeon HD4850 512MB DDR3 2DVI/HDCP PCI-Express Video Card