I want to upgrade to Vista home premium from windows xp pro, how much ram do i need and what is recommended?

Heres what my comp has now...
Windows Xp Pro
Pentium 4 Processor with Hyper-threading technology
250gb Hard Drive
1.5gb Ram
128mb ati radien graphics card

Do I need more Ram or anything like that? Will Vista run witohut lagging on my computer. Thanks!
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