Well I finished Stalker Shadow of Chernobyl with a wrong end, so I'm still playing both
I don't know how Clear Sky might be on XP but... what i see is nice, day and night lasts longer, people can bring you somewhere so you don't have to walk a whole map before getting where you want. The money got a sense now...
Well I like it but can only play it on Medium graphics but full dynamic range

My PC is good, but my graphic card cant take it at very high. Too bad...
My pc:
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System Information
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Operating System: Windows Vista™ Ultimate (6.0, Build 6001) Service Pack 1 (6001.vistasp1_gdr.080425-1930)
BIOS: Phoenix - AwardBIOS v6.00PG
Processor: AMD Athlon 64 X2 Dual Core Processor 5000+ (2 CPUs), ~2.6GHz
Memory: 4094MB RAM
Page File: 1499MB used, 6866MB available
DirectX Version: DirectX 10
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Display Devices
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Card name: ATI Radeon HD 2600 XT
Manufacturer: ATI Technologies Inc.
Display Memory: 2034 MB
Dedicated Memory: 243 MB
Shared Memory: 1791 MB