Question relating to RAM and HDD

hello  techwizs'

i have a 2007 yr desktop running on:

MOBO- Conroe 1333-d667( http://www.asrock.com/mb/Intel/ConRoe1333-D667%20R1.0/ )
Memory- transcend DDRII-667 DIMM 2 sticks of 1GB each
Processor- intel pentium e2140 @1.60ghz (fsb 800mhz)
HDD- WD 80GB SATA

1) i am willing to replace one memory stick with a new 2GB of same make with same frequency etc. my mobo limit is 4gb max. will it create any complication for me. in cmos i can see the memory modules are clocked at 333 mhz. other than this can u pls suggestany other solution for better performance. i use this pc mostly for gaming and lil bit of office use.

2) i wont to replace the 80 GB HDD with a new Seagate Barracuda 500 GB Desktop Internal Hard Drive (ST500DM002)
http://www.flipkart.com/seagate-barracuda-500-gb-deskto...

or with a

Seagate Momentus 320 GB Internal Hard Drive (ST9320325AS)
http://www.flipkart.com/seagate-momentus-320-gb-interna...

the retailer told me that if i install 500gb my pc will get slower. is he correct. if thats true will the addition of one more gb to RAM solve the problem. seriously is there any link between hdd and ram.

i am very dumb in these technical matters ,will be grateful if u could help me out....